Optical glass, preform for precision press molding, and optical element

ABSTRACT

The present invention relates to an optical glass with a high refractive index and good precision press moldability, and a preform for precision press molding and an optical element that are comprised of the optical glass. The present invention further relates to a method of manufacturing an optical element, a lens unit being equipped with an optical element and an image pickup device being equipped with a lens unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. Provisional Application No. 61/149,493 filed on Feb. 3, 2009, which is expressly incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an optical glass, a preform for precision press molding, and an optical element. More particularly, the present invention relates to an optical glass with a high refractive index and good precision press moldability, and a preform for precision press molding and an optical element that are comprised of the optical glass.

BACKGROUND ART

Correction of chromatic aberration in image pickup optical systems, typified by camera lenses, is achieved by combining a high dispersion lens and a low dispersion lens. In recent years, the trend has been to attempt to achieve greater functionality and compactness through the use of optical lenses with higher refractive indexes for both high and low dispersion lenses within the feasible scope of correction of chromatic aberration.

In addition to manipulating the optical characteristics of the glass constituting a lens, it is also possible to achieve greater functionality and compactness in optical systems by rendering the optical functional surfaces of the lens asymmetric. The precision press molding method is suited to the large-scale production of such lenses.

To improve the productivity of the precision press molding method, it is necessary to first achieve a certain degree of moldability to ensure the productivity of glass materials or preforms for precision press molding. That is, a certain viscosity of glass at a temperature at which crystals do not precipitate, or a certain degree of resistance to devitrification at a temperature at which the glass is of moldable viscosity, is required.

On that basis, it is also necessary to enhance the productivity of precision press molding. Expensive mold materials that are resistant to heat and highly rigid, such as SiC and superhard materials, are employed in the precision press molding method. Pressing molds with molding surfaces that have been precision processed into shapes the reverse of the shapes of the optically functional surfaces of optical elements are employed. As a result, the service lifetime of the pressing mold greatly affects production costs and productivity. Thus, effort has been expended on employing glasses with a low glass transition temperature and sag temperature to lower the temperature during press molding and prevent the thermal deterioration of the pressing mold.

Patent Document 1 discloses an optical glass having a low glass transition temperature (Tg), a high refractive index, low dispersion, and good internal glass quality, that is suited to reheat press molding and/or precision press molding. This optical glass is described as having optical constants within the specific ranges indicated in FIG. 1 of Patent Document 1, a low glass transition temperature (Tg), and good internal glass quality that are achieved by the incorporation of prescribed quantities of SiO₂, B₂O₃, La₂O₃, Gd₂O₃, Li₂O, and F. Thus it is suited to reheat press molding and/or precision press molding.

The optical glass described in Patent Document 2 is a known optical glass having a composition similar to that described in Patent Document 1. An SiO₂—B₂O₃—La₂O₃—ZnO—ZrO₂—F system optical glass with a high refractive index and low dispersion is disclosed in Patent Document 2. This optical glass is described has having a high refractive index, low dispersion, and good devitrification stability. However, there is no description regarding reheat press molding or precision press molding in Patent Document 2. Patent Document 3 describes an SiO₂—B₂O₃—La₂O₃—Gd₂O₃—Li₂O—F system low transition temperature (Tg) glass preform material having a high refractive index and low dispersion within a prescribed range for use in precision press molding, and an optical glass suited to precision press molding.

-   [Patent Document 1] Japanese patent publication 2005-170782 -   [Patent Document 2] Japanese patent publication shou 59-169952 -   [Patent Document 3] WO2004/015978 -   Patent Documents 1 to 3 are expressly incorporated herein by     reference in their entirety

The differences between Patent Documents 1 and 2 are as follows. The optical glass described in Patent Document 1 contains essentially no As₂O₃, while all the glasses specifically disclosed in Patent Document 2, with but one exception, contain As₂O₃. In Patent Document 1, As₂O₃ is eliminated as a component that greatly burdens the environment.

The SiO₂—B₂O₃—La₂O₃—ZnO—ZrO₂—F system optical glasses described in above-cited Patent Documents 1 and 2 are good optical glasses with high refractive indexes and low dispersion. However, the glasses described in Patent Document 1 comprise a relatively large quantity of expensive Yb₂O₃, or have a high liquidus temperature, which presents problems in terms of moldability. Many of the glasses disclosed as examples have glass transition temperatures exceeding 600° C., and are not necessarily suitable as optical glasses for precision press molding. The optical glasses of Patent Document 2 have a high glass transition temperature, and cannot be considered suitable for precision press molding. The optical glasses described in above-cited Patent Document 3 present such problems as inadequate thermal stability.

Accordingly, an object of the present invention is to provide an optical glass that solves the above problems and is suited to precision press molding.

SUMMARY OF THE INVENTION

The optical glass of the present invention is comprised of two basic forms specified from different perspectives.

The first basic form of the present invention provides an optical glass having a high refractive index and low dispersion characteristics, exhibiting good thermal stability, with a low glass transition temperature, that is suited to precision press molding.

The optical glass of the first basic form of the present invention is an optical glass characterized by comprising, denoted as cationic percentages,

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; as well as O and F, with the F/O molar ratio of the F content to the O content being 0.01 to 0.30.

The second basic form provides a high quality optical glass containing B, O, and F as components, volatizing little in melt form, and having characteristics such as optical characteristics that vary little.

The optical glass of the second basic form of the present invention is an optical glass comprising glass components in the form of cations including B and anions including O and F, wherein the value of B_(B)—(B_(O)—B_(B)) falls within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O).

The present invention further comprises four forms, denoted by A to D below, that are included in the first and second basic forms. The four forms of A to D relate to optical glasses that satisfy the conditions of the above-described first and second basic forms and have refractive indexes nd and Abbé numbers ν(nu)d that lie in mutually different regions.

The optical glass of Form A is an optical glass comprising,

denoted as cationic percentages:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 20 percent; ZrO₂ 0 to 4.5 percent; TaO_(2.5) 0 to 7 percent; YbO_(1.5) 0 to 0.5 percent; GeO₂ 0 to 5 percent; as well as O and F,

with the F/O molar ratio of the F content to the O content being 0.01 to 0.30;

the combined sum of the ZnO content, three times the ZrO₂ content, and five times the TaO_(2.5) content (ZnO+3ZrO₂+5TaO_(2.5)) being 40 percent or less;

the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater;

the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to +20, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O);

the refractive index nd being 1.75 to 1.81; and

the Abbé number νd being 48 to 52.

The optical glass of Form B is an optical glass comprising,

denoted as cationic percentages:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 25 percent; ZrO₂ 8 percent or less; TaO_(2.5) 7 percent or less; YbO_(1.5) 0 to 3 percent; GeO₂ 0 to 5 percent; as well as O and F,

with the F/O molar ratio of the F content to the O content being 0.01 to 0.30;

the ratio of the ZrO₂ content to the combined content of ZrO₂ and TaO_(2.5)(ZrO₂/(ZrO₂+TaO_(2.5))) being 0 to 0.7;

the combined sum of ZrO₂ and TaO_(2.5)(ZrO₂+TaO_(2.5)) being 3 to 10;

the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater;

the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to +20, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O);

the refractive index nd being 1.79 to 1.835; and

the Abbé number νd being 42 to 48.

The optical glass of Form C is an optical glass comprising,

denoted as cationic percentages:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 15 percent; ZrO₂ 5 percent or less; TaO_(2.5) 3 percent or less; YbO_(1.5) 0 to 3 percent; GeO₂ 0 to 5 percent; as well as O and F,

with the F/O molar ratio of the F content to the O content being 0.01 to 0.30;

the combined sum of the ZnO content, three times the ZrO₂ content, and five times the TaO_(2.5) content (ZnO+3ZrO₂+5TaO_(2.5)) being 15 percent or less;

the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater;

the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O);

the refractive index nd being 1.675 to 1.76; and

the Abbé number νd being 51 to 58.

The optical glass of Form D is an optical glass comprising,

denoted as cationic percentages:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 10 percent; ZnO 0 to 28 percent; ZrO₂ 0 to 8 percent; TaO_(2.5) 0 to 10 percent; TiO₂ 0 to 8 percent; NbO_(2.5) 0 to 8 percent; WO₃ 0 to 10 percent; YbO_(1.5) 0 to 3 percent; GeO₂ 0 to 5 percent; as well as O and F,

with the F/O molar ratio of the F content to the O content being 0.01 to 0.30;

the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater;

the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to 0, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O);

the refractive index nd being 1.825 to 1.90; and

the Abbé number νd being 35 to 43.

The first basic form of the present invention provides an optical glass characterized by having a high refractive index, low dispersion, good thermal stability, and a low glass transition temperature, that is suited to precision press molding. The second basic form of the present invention provides a quality optical glass comprising glass components in the form of B, O, and F, with little volatility in a melted state, that exhibits little variation in characteristics such as optical characteristics.

The four forms A to D of the present invention provide optical glasses that satisfy the conditions of the first and second basic forms. Thus, they exhibit the effects of the invention that are found in the first and second basic forms of the present invention, while additionally having the refractive indexes nd and Abbé numbers νd corresponding to their individual forms.

Form A of the present invention provides an optical glass having a refractive index nd of 1.75 to 1.81 and an Abbé number of 48 to 52; Form B of the present invention provides an optical glass having a refractive index nd of 1.79 to 1.835 and an Abbé number of 42 to 48; Form C of the present invention provides an optical glass having a refractive index nd of 1.675 to 1.76 and an Abbé number of 51 to 58; and Form D of the present invention provides an optical glass having a refractive index nd of 1.825 to 1.90 and an Abbé number of 35 to 43, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 The numeric values recorded near the various plots in FIG. 1 denote the melting time in unit of hours.

FIG. 2 shows the change in the refractive indexes of Examples 32 and 33 in Table 3 (Form A), with the horizontal axis denoting the melting time and the vertical axis denoting the refractive index nd.

FIG. 3 shows the change in specific gravity of Examples 32 and 33 in Table 3 (Form A), with the horizontal axis denoting the melting time and the vertical axis denoting specific gravity.

FIG. 4 shows the change in the glass transition temperature of Example 32 in Table 3 (Form A), with the horizontal axis denoting the melting time and the vertical axis denoting the glass transition temperature Tg.

FIG. 5 shows the change in Tg resulting from change in 2LiO_(0.5)+ZnO+(F/2), with the horizontal axis denoting the value of 2LiO_(0.5)+ZnO+(F/2) and the vertical axis denoting the glass transition temperature Tg.

FIG. 6 shows change in the inverse of kinematic viscosity and change in viscosity corresponding to change in temperature in an example (Example 8 (nd=1.72686, νd=54.41) of Form C (Table 5)) of the optical glass of the present invention, with the horizontal axis denoting temperature, the left vertical axis denoting the viscosity of the glass, and the right vertical axis denoting the inverse of kinematic viscosity, an index of the flow rate.

FIG. 7 shows microscopic photographs of the various glasses, following holding, in the comparative examples.

FIG. 8 shows the results (photographs) of a comparison experiment of wet backwordly.

FIG. 9 shows transmittance curves for glasses reproduced from Patent Documents 1 and 3.

FIG. 10 shows change in weight reduction (volatilization) due to change in the F content for optical glasses of the present invention of Forms A to D, with the horizontal axis denoting the F content and the vertical axis denoting change in weight.

FIG. 11 shows the relation between the D value and the change in weight for glasses having an F content of 7 to 9 percent, with the horizontal axis denoting the D value and the vertical axis denoting change in weight.

FIG. 12 shows the relation between the D value and the change in weight for glasses having an F content of 14 to 16 percent, with the horizontal axis denoting the D value and the vertical axis denoting change in weight.

FIG. 13 is a combination of FIGS. 11 and 12.

MODES FOR CARRYING OUT THE INVENTION

The optical glass of the present invention is described in greater detail below.

The optical glass of the first basic form and the optical glass of the second basic form of the present invention share the fact that they comprise the glass components of B as a cation and O and F as anions. There are thus cases where their compositions and physical properties overlap. That is, the optical glass of the first basic form can satisfy the conditions prescribed for the optical glass of the second basic form, and the optical glass of the second basic form can satisfy the conditions prescribed for the optical glass of the first basic form.

Tables 1-1 to 1-29 and 2-1 to 2-18 give desirable composition ranges and desirable characteristics for the optical glass of the first basic form. An optical glass having any combination of the composition ranges and characteristic ranges indicated in these tables is possible in the present invention.

Any optical glass optionally combining a composition range and a characteristic range given in any of Tables 1-1 to 1-29 and 2-1 to 2-18 corresponds to desirable optical glass of the second basic form.

Tables 1-1 to 1-29 and 2-1 to 2-18 record desirable composition ranges and desirable characteristics for the four forms of A to D. For Forms A to D, any combination of desirable forms of the first form and the second form is possible so long as it falls within the range of one of the forms.

As set forth above, Form A is a glass that is suited to realizing optical constants in the form of a refractive index nd of 1.75 to 1.81 and an Abbé number νd of 48 to 52.

The fact that Form A, as denoted by cationic percentages, is an optical glass comprising:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; as well as O and F, with the F/O molar ratio of the F content to the O content being 0.01 to 0.30, is shared with optical glasses of the first basic form of the present invention. These compositions will be described further below with reference to Tables 1 and 2. Satisfying these conditions yields an optical glass having characteristics of a high refractive index and low dispersion, good thermal stability, and a low glass transition temperature that is suited to precision press molding.

Further, the fact that Form A is an optical glass comprising the glass components of cations including B and anions including O and F, and an optical glass in which the value of B_(B)—(B_(O)—B_(B)) falls within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O), is shared with optical glasses of the second basic form of the present invention. Satisfying these conditions yields a quality optical glass comprising glass components in the form of B, O, and F, with little volatilization in a melted state, exhibiting little variation in various characteristics such as optical characteristics. However, in Form A, the upper limit of B_(B)—(B_(O)—B_(B)) is +20. The reasons for this are as follows. In Form A, when large numbers of components with high melting points, such as rare earth components, Zr, and Ta, are included among the glass components to increase the refractive index, the liquidus temperature increases and the temperature at which the glass flows out tends to rise. Accordingly, this upper limit, an indicator of the ratio of volatilizable bonding in the present glass system, is set to inhibit volatilization of glass components at relatively high temperatures.

The fact that

SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 20 percent; and GeO₂ 0 to 5 percent are also incorporated is shared with optical glasses of the first and second basic forms of the present invention. A description of these compositions will be given further below with reference to Tables 1 and 2.

The fact that

ZrO₂ 0 to 4.5 percent; TaO_(2.5) 0 to 7 percent; and YbO_(1.5) 0 to 0.5 percent are incorporated differs from the optical glasses of the first and second basic forms of the present invention.

ZrO₂ is set to 0 to 4.5 percent because when the ZrO₂ content exceeds 4.5 percent, the dispersion tends to increase and the liquidus temperature tends to rise. The upper limit of the ZrO₂ content is desirably 3.8 percent, preferably 3.4 percent, more preferably 3 percent, still more preferably 2.7 percent, and yet more preferable upper limit is 2.5 percent.

TaO_(2.5) is set to 0 to 7 percent because when the content of TaO_(2.5) exceeds 7 percent, it becomes difficult to maintain desired dispersion and good stability. The upper limit of the TaO_(2.5) content is desirably 5 percent or lower, preferably 4 percent or lower, more preferably 3 percent, still more preferably 2 percent, yet more preferably 1.5 percent, and even more preferably, 1.2 percent. The lower limit of the TaO_(2.5) content is desirably 0.1 percent, preferably 0.3 percent, more preferably 0.5 percent, and still more preferably, 0.6 percent.

ZrO₂ is one of the glass starting materials that melts the most poorly and requires a high temperature for homogenization. This causes the volatilization of a large amount of fluorine in the process of melting and homogenizing the starting materials of the glass of the present invention, which include fluorine. Thus, the incorporation of large quantities of this component is undesirable from the perspective of suppressing the variation in glass characteristics that accompany volatilization.

YbO_(1.5) is set to 0 to 0.5 percent because of high cost and because the incorporation of an excessive quantity increases the liquidus temperature, indirectly fostering the volatilization of glass components and compromising the homogeneity and molding characteristics of the glass. Further, this component absorbs radiation in the infrared region, rendering the glass unsuitable for use in highly sensitive optical systems of which sensitivity characteristics in the near infrared range are required, such as high-precision video cameras and surveillance cameras. In particular, the optical lens of Form A is suited to the greatest range of optical design applications among all of the forms. To permit use in these optical systems, the quantity of YbO_(1.5) is desirably limited.

(ZnO+3ZrO₂+5TaO_(2.5)) is set to 40 percent or less to maintain low dispersion. (2LiO_(0.5)+ZnO+(F/2)) is set to 20 percent or more to lower the (maintain a low) glass transition temperature.

In particular, as set forth above, Form A is an optical glass having optical constants in the form of a refractive index nd of 1.75 to 1.81 and an Abbé number νd of 48 to 52. Thus, to obtain an optical glass satisfying both this refractive index nd and Abbé number νd, it suffices to divide the glass components into the following categories and adjust the contents thereof. That is, it suffices to adjust: (1) the total quantity NWF of glass forming components, such as BO_(1.5), SiO₂, and AlO_(1.5), an increase in the content of which has the effect of reducing the refractive index and increasing the Abbé number; (2) the total quantity Σ(SIGMA)RE of rare earth components, such as LaO_(1.5), GdO_(1.5), and YO_(1.5), an increase in the content of which has the effect of increasing the refractive index and slightly reducing the Abbé number, thereby increasing the refractive index and lowering the dispersion of the glass; (3) the total quantity (ZrO₂+TaO_(2.5)) of components, such as ZrO₂ and TaO_(2.5), an increase in the content of which has the effect of greatly increasing the refractive index and slightly reducing the Abbé number; (4) the total quantity of high refractive index, high dispersion elements, such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5), an increase in the content of which has the effect of greatly increasing the refractive index and greatly reducing the Abbé number; (5) the total quantity of monovalent and divalent components, such as LiO_(0.5) and ZnO, an increase in the content of which has the effects of lowering the melting temperature of the glass starting materials, enhancing the stability of the glass, lowering the liquidus temperature, lowering the glass transition temperature, and the like; and (6) the total quantity of components other than those listed above. Among these, the adjustment of (1) to (4) above is effective for adjusting optical characteristics.

Specifically, the following applies for important components (1) to (3) of the present invention. The upper limit of (1) NWF is desirably 63 percent, preferably 60 percent, more preferably 58 percent, still more preferably 57 percent, still more preferably 56 percent, still more preferably 55 percent, and yet more preferably, 54 percent. Additionally, the lower limit of NWF is desirably 41 percent, preferably 44 percent, more preferably 46 percent, still more preferably 47 percent, still more preferably 48 percent, still more preferably 49 percent, and yet more preferably, 50 percent. The upper limit of (2) ΣRE is desirably 38 percent, preferably 36 percent, more preferably 35 percent, still more preferably 34 percent, still more preferably 33 percent, still more preferably 32 percent, and yet more preferably, 31 percent. Additionally, the lower limit of ΣRE is desirably 20 percent, preferably 22 percent, more preferably 23 percent, still more preferably 24 percent, still more preferably 25 percent, still more preferably 26 percent, and yet more preferably, 27 percent. The upper limit of (3) (ZrO₂+TaO_(2.5)) is desirably 8 percent, preferably 7 percent, more preferably 6 percent, still more preferably 5 percent, still more preferably 4.5 percent, still more preferably 4 percent, and yet more preferably, 3.5 percent. Additionally, the lower limit of (ZrO₂+TaO_(2.5)) is desirably 0 percent, preferably 1 percent, more preferably 2 percent, still more preferably 2.5 percent, and yet more preferably, 3 percent.

The quantities of ΣRE and NWF can be adjusted by referencing the ratio of ΣRE and NWF, namely, ΣRE/NWF. The upper limit of ΣRE/NWF is desirably 0.9, preferably 0.85, still more preferably 0.80, still more preferably 0.75, still more preferably 0.70, still more preferably 0.65, and yet more preferably, 0.60. Additionally, the lower limit of ΣRE/NWF is desirably 0.50, preferably 0.45, more preferably 0.40, still more preferably 0.35, and yet more preferably, 0.30.

Additional conditions are as follows. To lower the liquidus temperature by mixing rare earth components, thereby raising (maintaining) the stability of a glass with relatively good optical characteristics, it is desirable to pay attention to the ratio of LaO_(1.5) to the total quantity of rare earth elements ΣRE, namely LaO_(1.5)/ΣRE. In Form A, which has the above-described desirable component structure, the upper limit of LaO_(1.5)/ΣRE is desirably 0.75, preferably 0.7, more preferably 0.65, still more preferably 0.6, still more preferably 0.57, still more preferably 0.55, still more preferably 0.53, and yet more preferably, 0.52. Additionally, the lower limit of LaO_(1.5)/ΣRE is desirably 0.30, preferably 0.4, more preferably 0.45, still more preferably 0.48, and yet more preferably, 0.49.

In particular, in Form A, in addition to essential rare earth components, it is desirable to preferentially incorporate Zr and Ta from among the high refractive index and high dispersion elements of Zr, Ta, Ti, Nb, W, and Bi. It is preferable to preferentially employ Zr over Ta to achieve both a high refractive index and low dispersion. Thus, optimization of the quantity of Zr incorporated in Form A is extremely effective for enhancing the optical characteristics and molding properties of the glass.

TiO₂ and NbO_(2.5) can be further incorporated into Form A as optional components. TiO₂ and NbO_(2.5) function to increase the refractive index while maintaining low dispersion. However, since they also increase dispersion, TiO₂ is set to 0 to 3 percent and NbO_(2.5) to 0 to 3 percent.

Other high refractive index and high dispersion components can be incorporated to the extent that the effect of the present invention is not spoiled. The upper limit of the total quantity of high refractive index and high dispersion components such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5) is desirably 6 percent, preferably 5 percent, more preferably 4 percent, still more preferably 3 percent, still more preferably 2 percent, and yet more preferably, 1 percent. Optimally, they are not incorporated at all.

The incorporation of 2 percent or more of SiO₂ and maintenance of the ratio (SiO₂/BO_(1.5)) at 0.10 or higher are desirable in Form A. This makes it possible to increase the viscosity of the glass melt and enhances the inhibition of striae during molding.

The refractive index nd and the Abbé number νd in Form A desirably satisfy equation (1) below. The relation between the refractive index nd and the Abbé number νd is described further below. nd−(2.25−0.01×νd)≧−0.01  (1)

Form B is a glass suited to realizing optical constants in the form of a refractive index nd of 1.79 to 1.835 and an Abbé number νd of 42 to 48.

The facts that Form B is an optical glass comprising, denoted as cationic percentages:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; as well as O and F, and that the F/O molar ratio of the F content to the O content is 0.01 to 0.30, are shared with optical glass of the first basic form of the present invention. A description of these compositions will be given further below with reference to Tables 1 and 2. Satisfying these conditions yields an optical glass having high refractive index and low dispersion characteristics, good thermal stability, and a low glass transition temperature, that is suited to precision press molding.

The fact that Form B is an optical glass comprising glass components in the form of cations including B and anions including O and F, such that the value of B_(B)—(B_(O)—B_(B)) falls within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O), is shared with the optical glass of the second basic form of the present invention. Satisfying these conditions yields a quality optical glass containing glass components in the form of B, O, and F, that undergoes little volatilization in a melted state and exhibits little variation in various characteristics such as optical characteristics. However, in Form B, the upper limit of B_(B)—(B_(O)—B_(B)) is +20. The reason for this is as follows. To raise the refractive index of Form B relative to that of Form C, large quantities of components with high melting points, such as rare earth elements, Zr, and Ta, are included among the glass components. This increases the liquidus temperature, and causes the temperature at which the glass flows out to increase. Accordingly, an upper limit, indicating the ratio of volatilizable bonding in the glass system, is set to inhibit volatilization of glass components at relatively high temperatures.

The fact of comprising

SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; and GeO₂ 0 to 5 percent is also shared with the optical glasses of the first and second basic forms of the present invention. The compositions thereof will be described further below with reference to Tables 1 and 2.

The fact of comprising

ZnO 0 to 25 percent; ZrO₂ 8 percent or less; TaO_(2.5) 7 percent or less; and YbO_(1.5) 0 to 3 percent is a difference with the optical glasses of the first and second basic forms of the present invention.

ZnO is set to 0 to 25 percent because although the introduction of a large quantity of ZnO increases the refractive index somewhat in terms of optical characteristics, there is a problem in that dispersion increases (νd decreases). However, the object of Form B is to obtain a glass having a higher refractive index and a greater dispersion range than Form A, namely nd=1.79 to 1.835 and νd=42 to 48. Thus, the content permitted is greater than in the glass of Form A.

Both ZrO₂ and TaO_(2.5) are components that raise the refractive index. The incorporation of suitable quantities thereof stabilizes the glass, lowers the liquidus temperature, and increases the viscosity of the glass melt. The quantity of ZrO₂ is set to 8 percent or less because when the content of ZrO₂ exceeds 8 percent, the liquidus temperature tends to increase. The upper limit of the ZrO₂ content is desirably 7 percent, preferably 6 percent, more preferably 5 percent, still more preferably 4.5 percent, and yet more preferably, 4 percent. The lower limit of the ZrO₂ content is desirably 0.5 percent, preferably 1 percent, and more preferably, 1.5 percent.

TaO_(2.5) is set to 7 percent or less because when the content of TaO_(2.5) exceeds 7 percent, it becomes difficult to maintain the desired dispersion and good stability. The upper limit of the TaO_(2.5) content is desirably 6 percent or less, preferably 5 percent. The lower limit of the TaO_(2.5) content is desirably 0.1 percent, preferably 0.3 percent, more preferably 0.5 percent, still more preferably 0.6 percent, still more preferably 1 percent, still more preferably 1.5 percent, still more preferably 1.8 percent, and yet more preferably, 2 percent.

YbO_(1.5) is set to 0 to 3 percent because of high cost and because the introduction of an excessive quantity increases the liquidus temperature, indirectly fostering volatilization of glass components and compromising the homogeneity and molding properties of the glass. Further, this component absorbs radiation in the infrared region, rendering the glass unsuitable for use in highly sensitive optical systems of which sensitivity characteristics in the near infrared range are required, such as high-precision video cameras and surveillance cameras. However, since the glass of Form B has a high refractive index, it can be effectively limited to applications such as compact digital cameras that do not receive light in the infrared range. Thus, YbO_(1.5) can be incorporated to an upper limit of 3 percent.

(ZrO₂/(ZrO₂+TaO_(2.5))) is limited to from 0 to 0.7 to optimize the distribution of Zr and Ta employed in Form B and lower the liquidus temperature, thereby yielding a highly stable glass. (ZrO₂+TaO_(2.5)) is set to 3 to 10 to achieve a higher refractive index by using more Zr and Ta than in Form A. The total quantity of (2LiO_(0.5)+ZnO+(F/2)) is set to 20 percent or more to lower the (keep a low) glass transition temperature.

To further improve the stability of the glass and lower the liquidus temperature in Form B, (ZrO₂/(ZrO₂+TaO_(2.5))) is desirably limited to 0.7 or lower. From the perspectives of improving the stability of the glass and lowering the liquidus temperature, the upper limit of (ZrO₂/(ZrO₂+TaO_(2.5))) is desirably 0.6, preferably 0.55, more preferably 0.50, and still more preferably, 0.45.

The lower limit of (ZrO₂/(ZrO₂+TaO_(2.5))) is desirably 0.00, preferably 0.10, more preferably 0.20, still more preferably 0.25, still more preferably 0.30, and yet more preferably, 0.35.

Instead of the above indicators, in glasses containing TaO_(2.5), (ZrO₂/TaO_(2.5)) is desirably limited to 2.5 or lower. From the perspectives of improving glass stability and lowering the liquidus temperature, the upper limit of (ZrO₂/TaO_(2.5)) is desirably 1.8, preferably 1.6, more preferably 1.4, still more preferably 1.3, still more preferably 1.2, and yet more preferably, 1.1. The lower limit of (ZrO₂+TaO_(2.5)) is desirably 0.1, preferably 0.2, more preferably 0.3, still more preferably 0.4, and yet more preferably, 0.5.

In particular, as set forth above, since Form B is an optical glass having optical constants in the form of a refractive index nd of 1.79 to 1.835 and an Abbé number νd of 42 to 48, to achieve an optical glass achieving both this refractive index nd and Abbé number νd, it suffices to classify the glass components as follows in the same manner as in Form A and adjust their contents. That is, it suffices to adjust: (1) the total quantity NWF of glass forming components, such as BO_(1.5), SiO₂, and AlO_(1.5), an increase in the content of which has the effect of reducing the refractive index and increasing the Abbé number; (2) the total quantity ΣRE of rare earth components, such as LaO_(1.5), GdO_(1.5), and YO_(1.5), an increase in the content of which has the effect of increasing the refractive index and slightly reducing the Abbé number, thereby increasing the refractive index and lowering the dispersion of the glass; (3) the total quantity (ZrO₂+TaO_(2.5)) of components, such as ZrO₂ and TaO_(2.5), an increase in the content of which has the effect of greatly increasing the refractive index and slightly reducing the Abbé number; (4) the total quantity of high refractive index, high dispersion elements, such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5), an increase in the content of which has the effect of greatly increasing the refractive index and greatly reducing the Abbé number; (5) the total quantity of monovalent and divalent components, such as LiO_(0.5) and ZnO, an increase in the content of which has the effects of lowering the melting temperature of the glass starting materials, enhancing the stability of the glass, lowering the liquidus temperature, lowering the glass transition temperature, and the like; and (6) the total quantity of components other than those listed above. Among these, the adjustment of (1) to (4) above is effective for adjusting optical characteristics.

Specifically, the following applies for important components (1) to (3) of the present invention. The upper limit of (1) NWF is desirably 60 percent, preferably 57 percent, more preferably 55 percent, still more preferably 54 percent, still more preferably 53 percent, still more preferably 52 percent, and yet more preferably, 51 percent. Additionally, the lower limit of NWF is desirably 35 percent, preferably 38 percent, more preferably 40 percent, still more preferably 41 percent, still more preferably 42 percent, still more preferably 43 percent, and yet more preferably, 44 percent. The upper limit of (2) ΣRE is desirably 42 percent, preferably 40 percent, more preferably 38 percent, still more preferably 37 percent, still more preferably 36 percent, still more preferably 35 percent, and yet more preferably, 34 percent. Additionally, the lower limit of ΣRE is desirably 20 percent, preferably 22 percent, more preferably 23 percent, still more preferably 25 percent, still more preferably 27 percent, still more preferably 28 percent, still more preferably 29 percent, and yet more preferably, 30 percent. The upper limit of (3) (ZrO₂+TaO_(2.5)) is desirably 10 percent, preferably 9 percent, more preferably 8 percent, still more preferably 7.5 percent, still more preferably 7 percent, still more preferably 6.5 percent, and yet more preferably, 6 percent. Additionally, the lower limit of (ZrO₂+TaO_(2.5)) is desirably 3 percent, preferably 3.5 percent, more preferably 4.0 percent, still more preferably 4.5 percent, and yet more preferably, 5.0 percent.

The quantities of ΣRE and NWF can be adjusted by referencing the ratio of ΣRE and NWF, namely, ΣRE/NWF. The upper limit of ΣRE/NWF is desirably 1.2, preferably 1.1, more preferably 1.0, still more preferably 0.95, still more preferably 0.9, still more preferably 0.85, and yet more preferably, 0.8. Additionally, the lower limit of ΣRE/NWF is desirably 0.35, preferably 0.40, more preferably 0.45, more preferably 0.50, still more preferably 0.55, still more preferably 0.60, and yet more preferably, 0.65.

Additional conditions are as follows. To lower the liquidus temperature by mixing rare earth components, thereby raising (maintaining) the stability of a glass with relatively good optical characteristics, it is desirable to pay attention to the ratio of LaO_(1.5) to the total quantity of rare earth elements ΣRE, namely LaO_(1.5)/ΣRE. In Form B, which has the above-described desirable component structure into which suitable quantities of Zr and Ta have been introduced in this manner, a ratio in which a large amount of La is employed is effective. That is, the upper limit of LaO_(1.5)/ΣRE is desirably 0.8, preferably 0.75, more preferably 0.70, still more preferably 0.67, still more preferably 0.65, still more preferably 0.63, and yet more preferably, 0.62. Additionally, the lower limit of LaO_(1.5)/ΣRE is desirably 0.30, preferably 0.4, more preferably 0.45, still more preferably 0.5, still more preferably 0.55, still more preferably 0.58, and yet more preferably, 0.59.

In particular, in Form B, in addition to essential rare earth components, it is desirable to preferentially incorporate Zr and Ta from among the high refractive index and high dispersion elements of Zr, Ta, Ti, Nb, W, and Bi. It is desirable to incorporate an optimal quantity of Zr while optimizing the content of Ta to achieve a higher refractive index and higher dispersion than in Form A. Thus, optimization of the quantities and ratio of Ta and Zr incorporated in Form B is extremely effective for enhancing the optical characteristics and molding properties of the glass.

TiO₂ and NbO_(2.5) can be further incorporated into Form B as optional components. TiO₂ and NbO_(2.5) function to increase the refractive index while maintaining low dispersion. However, since they also increase dispersion, TiO₂ is set to 0 to 3 percent and NbO_(2.5) to 0 to 3 percent.

Other high refractive index and high dispersion components can be incorporated to the extent that the effect of the present invention is not spoiled. The upper limit of the total quantity (TiO₂+NbO_(2.5)+WO₃+BiO_(1.5)) of high refractive index and high dispersion components such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5) is desirably 8 percent, preferably 6 percent, more preferably 5 percent, still more preferably 4 percent, still more preferably 3 percent, still more preferably 2 percent, and yet more preferably, 1 percent. Optimally, to achieve low dispersion, they are not incorporated at all.

In Form B, it is desirable for (2LiO_(0.5)+ZnO+(F/2)) to constitute 20 percent or more to lower the glass transition temperature.

In the optical glass of Form B, the refractive index nd and the Abbé number νd desirably satisfy equation (2) below. The relation between the refractive index nd and the Abbé number νd is described further below. nd−(2.25−0.01×νd)≧−0.01  (2)

Form C is a glass that is suited to realizing optical constants in the form of a refractive index nd of 1.675 to 1.76 and an Abbé number νd of 51 to 58.

The fact that Form C, as denoted by cationic percentages, is an optical glass comprising:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; as well as O and F, with the F/O molar ratio of the F content to the O content being 0.01 to 0.30, is shared with optical glasses of the first basic form of the present invention. These compositions will be described further below with reference to Tables 1 and 2. Satisfying these conditions yields an optical glass characterized by a high refractive index, low dispersion, good thermal stability, and a low glass transition temperature, that is suited to precision press molding.

Further, the fact that Form C is an optical glass comprising the glass components of cations including B and anions including O and F, and an optical glass in which the value of B_(B)—(B_(O)—B_(B)) falls within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O), is shared with optical glasses of the second basic form of the present invention. Satisfying these conditions yields a quality optical glass comprising glass components in the form of B, O, and F, with little volatilization in a melted state, exhibiting little variation in various characteristics such as optical characteristics.

The fact that

SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; and GeO₂ 0 to 5 percent are also incorporated is shared with optical glasses of the first and second basic forms of the present invention. A description of these compositions will be given further below with reference to Tables 1 and 2.

The fact that

ZrO₂ 5 percent or less; TaO_(2.5) 3 percent or less; and YbO_(1.5) 0 to 3 percent are incorporated differs from the optical glasses of the first and second basic forms of the present invention.

ZnO is set to 0 to 15 percent because when the ZnO content exceeds 15 percent, dispersion tends to increase, stability decreases, and the liquidus viscosity tends to diminish. The upper limit of the ZnO content is desirably 15 percent, preferably 10 percent, more preferably 8 percent, still more preferably 6 percent, still more preferably 4 percent, still more preferably 3 percent, and yet more preferably, 2 percent. The lower limit of the ZnO content is desirably 0 percent, preferably 0.5 percent, and more preferably, 1 percent. Thus, 2LiO_(0.5)/(2LiO_(0.5)+ZnO), which serves as a yardstick in the adjustment of components lowering the Tg, naturally assumes a somewhat high value. That is, the lower limit of 2LiO_(0.5)/(2LiO_(0.5)+ZnO) is desirably 0.65, preferably 0.7, more preferably 0.75, still more preferably 0.8, still more preferably 0.85, still more preferably 0.9, yet more preferably 0.95 and particularly preferably, 1.00.

ZrO₂ is set to 5 percent or less because when the ZrO₂ content exceeds 5 percent, dispersion tends to increase and the liquidus temperature tends to rise. The upper limit of the ZrO₂ content is desirably 5 percent, preferably 4 percent, more preferably 3 percent, still more preferably 2.5 percent, still more preferably 2 percent, still more preferably 1.5 percent, still more preferably 1 percent, and yet more preferably 0.5 percent. Optimally, no ZrO₂ is incorporated at all.

TaO_(2.5) is set to 3 percent or less because when the content of TaO_(2.5) exceeds 3 percent, it becomes difficult to maintain desired dispersion and good stability, and the cost of the starting materials increases. The upper limit of the TaO_(2.5) content is desirably 3 percent or lower, preferably 2 percent, more preferably 1 percent, and yet more preferably, 0.5 percent. Optimally, no TaO₂ is incorporated at all.

YbO_(1.5) is set to 0 to 3 percent because of high cost. Further, this component absorbs radiation in the infrared region, rendering the glass unsuitable for use in highly sensitive optical systems of which sensitivity characteristics in the near infrared range are required, such as high-precision video cameras and surveillance cameras. In particular, the optical glass of Form C can be of lower dispersion than that of Form A and the low dispersion of YBO_(1.5) can be utilized to prepare a low-dispersion glass in which, for example, νd=55 or higher. Thus, YbO_(1.5) can be incorporated up to a maximum of 3 percent.

In particular, as set forth above, Form C is an optical glass having optical constants in the form of a refractive index nd of 1.675 to 1.76 and an Abbé number νd of 51 to 58. Thus, to obtain an optical glass satisfying both this refractive index nd and Abbé number νd, it suffices to divide the glass components into the following categories in the same manner as in Form A and adjust the contents thereof. That is, it suffices to adjust: (1) the total quantity NWF of glass forming components, such as BO_(1.5), SiO₂, and AlO_(1.5), an increase in the content of which has the effect of reducing the refractive index and increasing the Abbé number; (2) the total quantity ΣRE of rare earth components, such as LaO_(1.5), GdO_(1.5), and YO_(1.5), an increase in the content of which has the effect of increasing the refractive index and slightly reducing the Abbé number, thereby increasing the refractive index and lowering the dispersion of the glass; (3) the total quantity (ZrO₂+TaO_(2.5)) of components, such as ZrO₂ and TaO_(2.5), an increase in the content of which has the effect of greatly increasing the refractive index and slightly reducing the Abbé number; (4) the total quantity of high refractive index, high dispersion elements, such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5), an increase in the content of which has the effect of greatly increasing the refractive index and greatly reducing the Abbé number; (5) the total quantity of monovalent and divalent components, such as LiO_(0.5) and ZnO, an increase in the content of which has the effects of lowering the melting temperature of the glass starting materials, enhancing the stability of the glass, lowering the liquidus temperature, lowering the glass transition temperature, and the like; and (6) the total quantity of components other than those listed above. Among these, the adjustment of (1) to (4) above is effective for adjusting optical characteristics.

Specifically, the following applies for important components (1) to (3) of the present invention. The upper limit of (1) NWF is desirably 75 percent, preferably 72 percent, more preferably 70 percent, still more preferably 68 percent, still more preferably 66 percent, still more preferably 64 percent, and yet more preferably, 62 percent. Additionally, the lower limit of NWF is desirably 48 percent, preferably 50 percent, more preferably 52 percent, still more preferably 53 percent, still more preferably 54 percent, still more preferably 55 percent, and yet more preferably, 56 percent. The upper limit of (2) ΣRE is desirably 35 percent, preferably 34 percent, more preferably 33 percent, still more preferably 32 percent, still more preferably 31 percent, still more preferably 30 percent, and yet more preferably, 29 percent. Additionally, the lower limit of ΣRE is desirably 14 percent, preferably 16 percent, more preferably 17 percent, still more preferably 18 percent, still more preferably 20 percent, still more preferably 22 percent, still more preferably 23 percent, and yet more preferably, 24 percent. The upper limit of (3) (ZrO₂+TaO_(2.5)) is desirably 5 percent, preferably 4 percent, more preferably 3.5 percent, still more preferably 3 percent, still more preferably 2.5 percent, still more preferably 2 percent, still more preferably 1.5 percent, still more preferably 1 percent, and yet more preferably, 0.5 percent. Optimally, (ZrO₂+TaO_(2.5)) constitutes 0 percent.

The quantities of ΣRE and NWF can be adjusted by referencing the ratio of ΣRE and NWF, namely, ΣRE/NWF. The upper limit of ΣRE/NWF is desirably 0.8, preferably 0.75, more preferably 0.70, still more preferably 0.65, still more preferably 0.60, still more preferably 0.55, and yet more preferably, 0.50. Additionally, the lower limit of ΣRE/NWF is desirably 0.15, preferably 0.20, more preferably 0.25, still more preferably 0.30, still more preferably 0.35, still more preferably 0.40, and yet more preferably, 0.45.

Additional conditions are as follows. To lower the liquidus temperature by mixing rare earth components, thereby raising (maintaining) the stability of a glass with relatively good optical characteristics, it is desirable to pay attention to the ratio of LaO_(1.5) to the total quantity of rare earth elements ΣRE, namely LaO_(1.5)/ΣRE. In Form C, which permits a total quantity of ΣRE that is smaller than that in Form A by limiting the quantities of Zr and Ta incorporated as set forth above, LaO_(1.5)/ΣRE assumes a wide range of values. That is, the upper limit of LaO_(1.5)/ΣRE is desirably 0.9, preferably 0.80, more preferably 0.70, still more preferably 0.60, still more preferably 0.57, and yet more preferably, 0.55. Additionally, the lower limit of LaO_(1.5)/ΣRE is desirably 0.30, preferably 0.4, more preferably 0.45, and yet more preferably, 0.48. In glasses in which the total amount of ΣRE is 20 or lower, LaO_(1.5)/ΣRE can be kept to 0.80 or higher.

In particular, in Form C, in addition to essential rare earth components, it is desirable to achieve a lower refractive index and lower dispersion than in Form A by not incorporating large amounts of the high refractive index and high dispersion components of Zr, Ta, Ti, Nb, W, and Bi. Limiting the quantities of Ta and Zr that are incorporated in Form C is extremely effective for enhancing the optical characteristics and molding properties of the glass.

TiO₂ and NbO_(2.5) can be further incorporated into Form C as optional components. TiO₂ and NbO_(2.5) function to increase the refractive index while maintaining low dispersion. However, since they also increase dispersion, TiO₂ is set to 0 to 2 percent and NbO_(2.5) to 0 to 3 percent.

Other high refractive index and high dispersion components can be incorporated to the extent that the effect of the present invention is not spoiled. The upper limit of the total quantity (TiO₂+NbO_(2.5)+WO₃+BiO_(1.5)) of high refractive index and high dispersion components such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5) is desirably 4 percent, preferably 3 percent, still more preferably 2 percent, and yet more preferably, 1 percent. Optimally, none of these components is incorporated at all.

(ZnO+3ZrO₂+5TaO_(2.5)) is set to 15 percent or less to maintain low dispersion.

(2LiO_(0.5)+ZnO+(F/2)) is set to 25 percent or more because the ratio of NWF in Form C is greater than in Form A and the glass transition point (Tg) tends to be high. The increase in the quantity of these Tg-lowering components yields a lower glass transition temperature.

To achieve lower dispersion in Form C than in Forms A, B, and D, the ratio of NWF to ΣRE is greater, and a large quantity of BO_(1.5) is incorporated. Thus, the upper limit of B_(B)—(B_(O)—B_(B)) of +60 is a greater value than in Forms A, B, and D. As a result, by keeping F/O, which denotes the content of F, an element contributing to volatilization, to 0.21 or lower, it is possible to compensate for the reduction in the volatilization-inhibiting effect due to a high upper limit of B_(B)—(B_(O)—B_(B)).

In the optical glass of Form C, the refractive index nd and the Abbé number νd desirably satisfy equation (3) below. The relation between the refractive index nd and the Abbé number νd is described further below. nd−(2.25−0.01×νd)≧−0.01  (3)

Form D is a glass suited to realizing optical constants in the form of a refractive index nd of 1.825 to 1.90 and an Abbé number νd of 35 to 43.

The facts that Form D is an optical glass comprising, denoted as cationic percentages:

BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; as well as O and F, and that the F/O molar ratio of the F content to the O content is 0.01 to 0.30, are shared with the optical glass of the first basic form of the present invention. A description of these compositions will be given further below with reference to Tables 1 and 2. Satisfying these conditions yields an optical glass with high refractive index and low dispersion characteristics, good thermal stability, and a low glass transition temperature, that is suited to precision press molding.

The fact that Form D is an optical glass comprising glass components in the form of cations including B and anions including 0 and F, such that the value of B_(B)—(B_(O)—B_(B)) falls within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O), is shared with the optical glass of the second basic form of the present invention. Satisfying these conditions yields a quality optical glass containing glass components in the form of B, O, and F, that undergoes little volatilization in a melted state and exhibits little variation in various characteristics such as optical characteristics. However, in Form D the upper limit of B_(B)—(B_(O)—B_(B)) is 0. The reason for this is as follows. To raise the refractive index of Form D higher than in Forms A, B, and C, large quantities of components with high melting points, such as rare earth elements, Zr, and Ta, are included among the glass components. This increases the liquidus temperature, making it necessary to reduce NWF components that lower the refractive index. Thus, the liquidus temperature increases, and the temperature at which the glass flows out tends to rise. Accordingly, to inhibit the volatilization of glass components at this higher temperature, the value of the upper limit, indicating the ratio of volatilizable bonding in the glass system, is strictly limited.

The fact of comprising

SiO₂ 0 to 20 percent; ZrO₂ 0 to 8 percent; TaO_(2.5) 0 to 10 percent; TiO₂ 0 to 8 percent; NbO_(2.5) 0 to 8 percent; WO₃ 0 to 10 percent; and GeO₂ 0 to 5 percent is also shared with the optical glasses of the first and second basic forms of the present invention. The compositions thereof will be described further below with reference to Tables 1 and 2.

The quantity of ZrO₂ is set to 8 percent or less because when the content of ZrO₂ exceeds 8 percent, dispersion and the liquidus temperature tend to increase. The upper limit of the ZrO₂ content is desirably 6 percent, preferably 5 percent, more preferably 4 percent, still more preferably 3.5 percent, and yet more preferably, 3 percent. The lower limit of the ZrO₂ content is desirably 0 percent, preferably 0.5 percent, more preferably 1 percent, still more preferably 1.5 percent, and yet more preferably, 2 percent.

The content of TaO_(2.5) is limited to 10 percent or less because when the content of TaO_(2.5) exceeds 10 percent, it becomes difficult to maintain the desired dispersion and good stability. The upper limit of the TaO_(2.5) content is desirably 9 percent, preferably 8 percent, more preferably 7 percent, and still more preferably, 6 percent. The lower limit of the TaO_(2.5) content is desirably 1 percent, preferably 2 percent, more preferably 3 percent, still more preferably 4 percent, and yet more preferably, 4.5 percent.

In particular, as set forth above, since Form D is an optical glass having optical constants in the form of a refractive index nd of 1.825 to 1.90 and an Abbé number νd of 35 to 43, to achieve an optical glass achieving both this refractive index nd and Abbé number νd, it suffices to classify the glass components as follows in the same manner as in Form A and adjust their contents. That is, it suffices to adjust: (1) the total quantity NWF of glass forming components, such as BO_(1.5), SiO₂, and AlO_(1.5), an increase in the content of which has the effect of reducing the refractive index and increasing the Abbé number; (2) the total quantity ΣRE of rare earth components, such as LaO_(1.5), GdO_(1.5), and YO_(1.5), an increase in the content of which has the effect of increasing the refractive index and slightly reducing the Abbé number, thereby increasing the refractive index and lowering the dispersion of the glass; (3) the total quantity (ZrO₂+TaO_(2.5)) of components, such as ZrO₂ and TaO_(2.5), an increase in the content of which has the effect of greatly increasing the refractive index and slightly reducing the Abbé number; (4) the total quantity of high refractive index, high dispersion elements, such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5), an increase in the content of which has the effect of greatly increasing the refractive index and greatly reducing the Abbé number; (5) the total quantity of monovalent and divalent components, such as LiO_(0.5) and ZnO, an increase in the content of which has the effects of lowering the melting temperature of the glass starting materials, enhancing the stability of the glass, lowering the liquidus temperature, lowering the glass transition temperature, and the like; and (6) the total quantity of components other than those listed above. Among these, the adjustment of (1) to (4) above is effective for adjusting optical characteristics.

Specifically, the following applies for important components (1) to (3) of the present invention. The upper limit of (1) NWF is desirably 49 percent, preferably 46 percent, more preferably 44 percent, still more preferably 43 percent, still more preferably 42 percent, still more preferably 41 percent, and yet more preferably, 40 percent. Additionally, the lower limit of NWF is desirably 32 percent, preferably 33 percent, more preferably 34 percent, still more preferably 35 percent, still more preferably 36 percent, and yet more preferably, 37 percent. The upper limit of (2) ΣRE is desirably 36 percent, preferably 35 percent, more preferably 34 percent, still more preferably 33 percent, still more preferably 32 percent, still more preferably 31 percent, and yet more preferably, 30 percent. Additionally, the lower limit of ΣRE is desirably 21 percent, preferably 22 percent, more preferably 23 percent, still more preferably 24 percent, still more preferably 25 percent, and yet more preferably, 26 percent. The upper limit of (3) (ZrO₂+TaO_(2.5)) is desirably 12 percent, preferably 11 percent, more preferably 10 percent, still more preferably 9.5 percent, still more preferably 9 percent, still more preferably 8.5 percent, and yet more preferably, 8 percent. Additionally, the lower limit of (ZrO₂+TaO_(2.5)) is desirably 3 percent, preferably 4 percent, more preferably 5 percent, still more preferably 5.5 percent, still more preferably 6 percent, still more preferably 6.5 percent, and yet more preferably, 7 percent.

The quantities of ΣRE and NWF can be adjusted by referencing the ratio of ΣRE and NWF, namely, ΣRE/NWF. The upper limit of ΣRE/NWF is desirably 1.2, preferably 1.1, more preferably 1.0, still more preferably 0.95, still more preferably 0.9, still more preferably 0.85, and yet more preferably, 0.8. Additionally, the lower limit of ΣRE/NWF is desirably 0.45, preferably 0.50, more preferably 0.55, still more preferably 0.60, still more preferably 0.65, and yet more preferably, 0.70.

Additional conditions are as follows. To lower the liquidus temperature by mixing rare earth components, thereby raising (maintaining) the stability of a glass with relatively good optical characteristics, it is desirable to pay attention to the ratio of LaO_(1.5) to the total quantity of rare earth elements ΣRE, namely LaO_(1.5)/ΣRE. In Form D, which has the above-described desirable component structure into which suitable quantities of Zr and Ta have been introduced in this manner, a ratio in which a large amount of La is employed is effective. That is, the upper limit of LaO_(1.5)/ΣRE is desirably 0.9, preferably 0.85, more preferably 0.8, still more preferably 0.77, and yet more preferably, 0.75. Additionally, the lower limit of LaO_(1.5)/ΣRE is desirably 0.5, preferably 0.55, still more preferably 0.60, still more preferably 0.65, and yet more preferably, 0.70.

In particular, in Form D, in addition to essential rare earth components, it is desirable to preferentially incorporate Zr and Ta from among the high refractive index and high dispersion elements of Zr, Ta, Ti, Nb, W, and Bi. It is also desirable to incorporate optimal quantities of Ti, Nb, W, and Bi to achieve a higher refractive index and greater dispersion than in Forms A, B, and C. Further, in addition to the quantities of Ta and Zr incorporated in Form D, based on the optical characteristics being sought, the incorporation of certain quantities of Ti, Nb, W, and Bi is extremely effective for enhancing the optical characteristics and molding properties of the glass.

The incorporation of WO₃ into Form D as an optional component is particularly effective to raise the refractive index and lower the liquidus temperature. The upper limit of WO₃ is desirably 10 percent, preferably 8 percent, more preferably 7 percent, still more preferably 6 percent, and yet more preferably, 5.5 percent. The lower limit is desirably 0 percent, preferably 1 percent, more preferably 2 percent, still more preferably 3 percent, and yet more preferably, 4 percent. The incorporation of 3 percent or more of WO₃, preferably 4 percent or more, is desirable when raising nd to 1.87 or higher.

TiO₂ and NbO_(2.5) can be further incorporated into Form D as optional components. TiO₂ and NbO_(2.5) function to increase the refractive index while maintaining low dispersion. However, since they also increase dispersion, TiO₂ is set to 0 to 5 percent and NbO_(2.5) to 0 to 5 percent.

Other high refractive index and high dispersion components can be incorporated to the extent that the effect of the present invention is not spoiled. The upper limit of the total quantity (TiO₂+NbO_(2.5)+WO₃+BiO_(1.5)) of high refractive index and high dispersion components such as TiO₂, NbO_(2.5), WO₃, and BiO_(1.5) is desirably 12 percent, preferably 11 percent, more preferably 10 percent, still more preferably 9 percent, still more preferably 8 percent, still more preferably 7 percent, and yet more preferably, 6 percent. From the perspectives of maintaining the stability of the glass and raising the refractive index without markedly increasing the liquidus temperature, the lower limit of the total quantity (TiO₂+NbO_(2.5)+WO₃+BiO_(1.5)) of high refractive index and high dispersion components is desirably 1 percent, preferably 2 percent, more preferably 3 percent, still more preferably 4 percent, and yet more preferably, 5 percent.

The fact of comprising:

LiO_(0.5) 0 to 10 percent; ZnO 0 to 28 percent; and YbO_(1.5) 0 to 3 percent differs from the optical glasses of the first and second basic forms of the present invention.

LiO_(0.5) is set to 0 to 10 percent because when the content of LiO_(0.5) exceeds 10 percent, the refractive index tends to decrease, the liquidus temperature rises, and the viscosity of the glass melt tends to decrease. Thus, the LiO_(0.5) content is set to 10 percent or lower. The upper limit of the LiO_(0.5) content is desirably 7 percent, preferably 5 percent, and more preferably, 4.5 percent. The lower limit of the LiO_(0.5) content is desirably 1 percent, preferably 2 percent, and more preferably, 2.5 percent.

ZnO is set to 0 to 28 percent because, in terms of optical characteristics, the refractive index increases somewhat and there is a problem in the form of increased dispersion (a reduction in νd) when a large quantity of ZnO is incorporated. In Form D, the object is to obtain a glass having a higher refractive index of nd=1.825 to 1.90 and a higher dispersion range of νd=35 to 43 than in Forms A, B, and C. Thus, the content permitted has been increased relative to what it is in the glasses of Forms A, B, and C. Thus, 2LiO_(0.5)/(2LiO_(0.5)+ZnO), which serves as a yardstick for adjusting components that lower the Tg, naturally assumes a lower value. That is, the upper limit of 2LiO_(0.5)/(2LiO_(0.5)+ZnO) is desirably 0.7, preferably 0.6, more preferably 0.5, and still more preferably, 0.4. The lower limit of 2LiO_(0.5)/(2LiO_(0.5)+ZnO) is desirably 0, preferably 0.05, more preferably, 0.1, still more preferably 0.15, still more preferably 0.2, and yet more preferably, 0.25.

YbO_(1.5) is set to 0 to 3 percent because of the high cost of YbO_(1.5) and because the incorporation of an excessive quantity increases the liquidus temperature, indirectly fostering the volatilization of glass components and compromising the homogeneity and molding characteristics of the glass. Further, this component absorbs radiation in the infrared region, rendering the glass unsuitable for use in highly sensitive optical systems of which sensitivity characteristics in the near infrared range are required, such as high-precision video cameras and surveillance cameras. However, the glass of Form D has a high refractive index, so its use can be effectively limited to applications such as compact digital cameras that do not receive light in the infrared range. YbO_(1.5) can be incorporated with 3 percent as the upper limit. The upper limit is set at 3 percent for YbO_(1.5) because of its high cost and the fact that it absorbs infrared radiation.

(2LiO_(0.5)+ZnO+(F/2)) is set to 20 percent or more to lower the glass transition temperature.

Further, in the optical glass of Form D, the refractive index nd and the Abbé number νd desirably satisfy equation (4). The relation between the refractive index nd and the Abbé number νd is described further below. nd−(2.25−0.01×νd)≧−0.01  (4)

The actions and effects of individual glass components will be described sequentially below. Tables 1-1 to 1-29 and 2-1 to 2-18 give the values of desirable upper limit and lower limit values for the ranges of physical properties and components. For both the upper limit and lower limit, the value indicated as number 1 is the outermost value of the numeric range. As the number indicated increases to 2, 3, 4, 5, 6, 7, or 8, the upper limit and the lower limit lie further inside the numeric range and increase in desirability. In the upper limit and lower limit fields of each table, the value indicated by the largest number is the most desirable value (upper limit value or lower limit value). The upper limit value and lower limit value of Tables 1-1 to 1-29 and 2-1 to 2-18 can be combined.

BO_(1.5) is an essential component in the optical glass of the present invention that forms a glass network. Incorporating BO_(1.5) into the optical glass of the present invention in a range of from 20 to 70 percent has the effect of enhancing glass stability. Incorporating 20 percent or more of BO_(1.5) has the effects of enhancing low dispersion and lowering the liquidus temperature. It is undesirable for the content of BO_(1.5) to exceed 70 percent because the refractive index decreases and the amount of volatilization tends to increase. From these perspectives, in the basic forms and in Forms A, B, C, and D of the present invention, the upper and lower limits of the BO_(1.5) content are suitably set as indicated below. In particular, from the perspective of increasing the refractive index, the upper limit in Form D is set to a lower value than in the basic forms. The lower limit in Form C is set to a higher value than in the basic forms to increase νd.

TABLE 1-1 BO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 70 65 64 69 45 2 65 60 57 65 43 3 60 51 52 60 42 4 55 48 48 55 41 5 51 46 45 53 40 6 48 44 43 51 39 7 46 42 41 50 38 8 44 39 9 42 Lower limit 1 20 25 40 24 2 30 28 24 42 26 3 34 30 28 44 28 4 36 34 31 46 29 5 38 36 33 30 6 39 38 34 7 40 39 35 8 40

SiO₂ is an optional component in the optical glass of the present invention that forms a glass network. SiO₂ can be incorporated into the optical glass of the present invention within a range of 0 to 20 percent. This has the effect of markedly improving glass stability. Incorporating more than 0 percent of SiO₂ has the effects of increasing the melt viscosity, lowering the liquidus temperature, and enhancing mechanical strength. An SiO₂ content exceeding 20 percent is undesirable in that the refractive index tends to decrease and the glass transition temperature tends to rise. From these perspectives, the upper and lower limits of the SiO₂ content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention.

TABLE 1-2 SiO₂ (Cation %) Form Basic A B C D Upper limit 1 20 17 16 17 15 2 17 15 13 15 10 3 15 13 11 13 8 4 13 12 10 12 7 5 12 11 9 11 6 6 11 5 Lower limit 1 0 2 0 2 1 2 2 4 2 4 2 3 5 5 4 5 3 4 7 7 5 7 5 8 8 6 8 6 9 9 7 9 7 10 8

The desirable range of the combined quantities of BO_(1.5) and SiO₂ is given below.

-   Basic forms: 32 to 74 percent, preferably 36 to 70 percent. -   Form A: 42 to 61 percent, 44 to 59 percent, 46 to 57 percent, 48 to     55 percent (increasing in desirability from left to right; identical     below); -   Form B: 39 to 57 percent, 41 to 55 percent, 43 to 53 percent, 45 to     51 percent; -   Form C: 50 to 72 percent, 52 to 70 percent, 54 to 68 percent, 56 to     66 percent; -   Form D: 30 to 46 percent, 32 to 45 percent, 34 to 44 percent, 35 to     43 percent.

AlO_(1.5) and GeO₂ are components that function to form a glass network in a manner equivalent to BO_(1.5) and SiO₂. When these components are incorporated into the optical glass of the present invention, the above total content ranges apply to AlO_(1.5) and GeO₂ as well as to BO_(1.5) and SiO₂. However, the effect of AlO_(1.5) is inferior to that of BO_(1.5) and SiO₂. Thus, the incorporation of BO_(1.5) and SiO₂ is preferred to the incorporation of AlO_(1.5). Since GeO₂ is expensive, the incorporation of BO_(1.5) and SiO₂ is preferable to the incorporation of GeO₂.

LaO_(1.5) is an essential component of the optical glass of the present invention that raises the refractive index. Incorporating LaO_(1.5) within a range of 6 to 30 percent in the optical glass of the present invention has the effect of raising the refractive index. The incorporation of 6 percent or more of LaO_(1.5) has the effects of maintaining low dispersion, enhancing chemical durability, and increasing mechanical strength. LaO_(1.5) has a lower melting point than other rare earth components, and is advantageous in that it can be incorporated in large quantity. The incorporation of more than 30 percent of LaO_(1.5) is undesirable in that the glass stability decreases, the liquidus temperature rises, and the glass transition temperature rises. From these perspectives, the upper and lower limits of the LaO_(1.5) content are suitably set as indicated below for the basic forms and forms A, B, C, and D of the present invention. In particular, from the perspective of raising the refractive index, the lower limit in Form D is set higher than in the basic forms.

TABLE 1-3 LaO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 30 25 24 27 2 25 22 26 22 26 3 22 20 24 20 24 4 20 18 23 18 22 5 18 17 22 17 6 17 16 21 16 7 16 Lower limit 1 6 8 8 6 14 2 8 9 11 8 16 3 9 10 14 9 17 4 10 11 16 10 18 5 11 12 18 11 6 12 19 12

GdO_(1.5) is an essential component of the optical glass of the present invention that raises the refractive index. Incorporating GdO_(1.5) into the optical glass of the present invention within a range of 4 to 25 percent has the effect of raising the refractive index. The incorporation of 4 percent or more of GdO_(1.5) has the effects of maintaining low dispersion, enhancing chemical durability, and increasing mechanical strength. GdO_(1.5) affords an advantage in that it lowers the melting point and increases moldability when mixed with other rare earth components. The incorporation of a quantity of GdO_(1.5) exceeding 25 percent is undesirable in that it decreases glass stability, raises the liquidus temperature, and raises the glass transition temperature. From such perspectives, the upper and lower limits of the GdO_(1.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, the upper limit in Form D is set to a lower value than in the basic forms from the perspective of setting a desirable LaO_(1.5)/ΣRE ratio that does not raise the liquidus temperature when high dispersion components are incorporated.

TABLE 1-4 GdO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 25 22 22 20 18 2 22 20 20 18 16 3 20 18 18 16 14 4 18 16 16 15 12 5 16 15 15 14 11 6 15 14 14 10 7 14 Lower limit 1 4 6 6 6 5 2 6 7 7 7 5.5 3 7 8 8 8 6 4 8 9 9 9 6.5 5 9 10 10 10 6 10

YO_(1.5) is an optional component in the optical glass of the present invention that raises the refractive index. The optical glass of the present invention can contain YO_(1.5) within a range of 0 to 10 percent, which has the effect of raising the refractive index. the incorporation of more than 0 percent of YO_(1.5) has the effects of maintaining low dispersion, enhancing chemical durability, and increasing mechanical strength. Further, YO_(1.5) affords an advantage in that it lowers the melting point and increases moldability when mixed with other rare earth components. Still further, YO_(1.5) has a high refractive index for its specific gravity, contributing to the reduction in weight of lenses comprised of the optical glass of the present invention. The incorporation of a quantity of YO_(1.5) exceeding 10 percent is undesirable because it decreases glass stability, raises the liquidus temperature, and raises the glass transition temperature. From such perspectives, the upper and lower limits of the YO_(1.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, the upper limit in Form D is set to a lower value than in the basic forms from the perspectives of setting a desirable LaO_(1.5)/ΣRE ratio that does not raise the liquidus temperature when high dispersion components are incorporated, and of preferentially employing rare earth components with high refractive indexes.

TABLE 1-5 YO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 10 7 7 7 5 2 7 5 5 5 3 3 5 4 4 4 2 4 4 3 3 3 1 5 3 2 2.5 0 6 2 7 1.5 8 1 9 0.5 10 0 Lower limit 1 0 0 0 2 0.5 0.5 0.5 3 1 1 1 4 1.5 1.5 5 2 2 6 2.5 2.5

LiO_(0.5) is an optional component of the optical glass of the present invention that lowers the glass transition temperature, sag temperature, and press molding temperature. LiO_(0.5) can be incorporated into the optical glass of the present invention in a range of from 0 to 20 percent, having the effect of markedly lowering the glass transition temperature. The incorporation of more than 0 percent of LiO_(0.5) yields a glass of higher density than other alkalis. As a result, it is possible to achieve an optical glass of higher refractive index than with other alkalis. The incorporation of more than 20 percent of LiO_(0.5) lowers the melt viscosity, raises the liquidus temperature, and as a result, decreases glass stability. Further, the incorporation of more than 20 percent of LiO_(0.5) is undesirable in that it lowers the refractive index. From such perspectives, the upper and lower limits of the LiO_(0.5) contents are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The upper limit of Form D is set to a value that is lower than those of the basic forms because high dispersion is permitted and Li and Zn are employed as components that lower the Tg.

TABLE 1-6 LiO_(0.5) (Cation %) Form Basic A B C D Upper limit 1 20 16 16 18 8 2 16 13 13 16 7 3 13 12 10 14 6 4 12 11 7 13 5 5 11 10 5 12 6 10 4 7 Lower limit 1 0 2 0 2 0 2 2 4 0.5 4 1 3 4 5 1 5 2 4 5 6 1.5 6 3 5 6 7 2 7 6 7 8 2.5 8 7 8 3 10

ZnO is an optional component in the optical glass of the present invention that lowers the glass transition temperature, sag temperature, and press molding temperature. The optical glass of the present invention can comprise ZnO within a range of 0 to 20 percent, thereby lowering the glass transition temperature. The incorporation of more than 0 percent of ZnO raises the refractive index, lowers the liquidus temperature, and suppresses volatilization more than Tg lowering components. The incorporation of more than 20 percent of ZnO is undesirable in that it increases dispersion (reduces νd) and lowers the nd relative to the specific gravity. From such perspectives, the upper and lower limits of the ZnO content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. From the perspective of increasing a low level of dispersion, the upper limit of Form C is set to a lower value than the basic forms. From the perspective of raising the refractive index, the lower limit of Form D is set to a value that is higher than that of the basic forms.

TABLE 1-7 ZnO (Cation %) Form Basic A B C D Upper limit 1 20 17 25 6 28 2 17 14 20 4 26 3 14 12 19 3 24 4 12 10 18 2 22 5 10 8 17 2 21 6 8 7 16 20 7 7 15 8 14 Lower limit 1 0 1 0 0 5 2 1 2 3 0.5 8 3 2 3 5 1 10 4 3 4 8 12 5 4 5 10 13 6 5 11 14 7 12 15

F is an essential component of the optical glass of the present invention that lowers the glass transition temperature, sag temperature, and press molding temperature. In the optical glass of the present invention, the molar ratio F/O of the F content to the O (oxygen) content is 0.01 to 0.30. Incorporating F within this range has the effect of lowering the glass transition temperature. Incorporating F in a quantity such that F/O is 0.01 or greater permits an increase in the quantity of rare earth components incorporated, enhances glass stability, and improves weatherability. The incorporation of F in a quantity such that F/O exceeds 0.30 is undesirable in that it promotes variation in characteristics due to volatilization, increases striae, lowers the refractive index, lowers the melt viscosity, and markedly diminishes the nd relative to the specific gravity. From such perspectives, the upper and lower limits of the F content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention.

TABLE 1-8 F/O (Molar ratio) Form Basic A B C D Upper limit 1 0.30 0.24 0.27 0.24 0.20 2 0.27 0.21 0.24 0.21 0.16 3 0.24 0.18 0.22 0.18 0.14 4 0.21 0.16 0.20 0.16 0.12 5 0.18 0.14 0.18 0.14 0.10 6 0.16 0.12 0.17 0.12 0.08 7 0.14 0.11 0.16 0.11 0.07 8 0.12 0.15 9 0.11 Lower limit 1 0.01 0.01 0.01 0.01 0.01 2 0.02 0.02 0.03 0.02 0.02 3 0.04 0.04 0.06 0.04 0.04 4 0.06 0.06 0.08 0.06 5 0.08 0.08 0.09 0.08 6 0.09 0.09 0.10 0.09

ZrO₂ is an optional component of the optical glass of the present invention that raises the refractive index. ZrO₂ can be incorporated in a quantity falling within a range of 0 to 8 percent in the optical glass of the present invention, having the effect of markedly raising the refractive index. The incorporation of a quantity of ZrO₂ of greater than zero percent has the effects of stabilizing the glass, lowering the liquidus temperature more than rare earth components, raising the viscosity, enhancing chemical durability, and increasing mechanical strength. The incorporation of a quantity of ZrO₂ exceeding 8 percent is undesirable in that it somewhat increases dispersion (reduces νd) and raises the glass transition temperature. From such perspectives, the upper and lower limits of ZrO₂ are suitably set as indicated below in the basic forms and Forms A, B, C, and D of the present invention. From the perspective of increasing νd, the upper limits of Forms A and C are set to lower values than in the basic forms.

TABLE 1-9 ZrO₂ (Cation %) Form Basic A B C D Upper limit 1 8 3.8 5.0 6.0 2 5 3.4 5 4.0 5.0 3 4.5 3.0 4.5 3.0 4.0 4 3.8 2.7 3.8 2.5 3.5 5 3.4 2.5 3.4 2.0 3.0 6 3 3 1.5 7 2.7 2.7 1.0 8 2.5 2.5 0.5 Lower limit 1 0 0 0 0 2 0.5 0.5 0.5 0.5 3 1 1 1 1 4 1.5 1.5 1.5 1.5 5 2 2 2 2

TaO_(2.5) is an optional component of the optical glass of the present invention that raises the refractive index. TaO_(2.5) can be incorporated into the optical glass of the present invention within a range of 0 to 10 percent, having the effect of markedly raising the refractive index. Incorporating a quantity of TaO_(2.5) of greater than zero percent has the effects of stabilizing the glass more than rare earth components, lowering the liquidus temperature more than rare earth components, raising the viscosity, enhancing chemical durability, and increasing mechanical strength. The incorporation of a quantity of TaO_(2.5) exceeding 10 percent is undesirable in that it increases dispersion (lowers νd) somewhat and increases the cost of the glass. From such perspectives, the upper and lower limits of the TaO_(2.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, from the perspective of lowering dispersion, the upper limits of Forms A and C are set to lower values than for the basic forms.

TABLE 1-10 TaO_(2.5) (Cation %) Form Basic A B C D Upper limit 1 10 5 10 3 10 2 7 4 8 2 9 3 5 3 7 1 8 4 4 2 6 0.5 7 5 3 1.5 5.5 0 6 6 2 1.2 4.8 7 1.5 4.3 8 1.2 4 Lower limit 1 0 0 0 0 2 0.1 0.1 0.5 1 3 0.3 0.3 1.2 2 4 0.5 0.5 1.8 3 5 0.6 0.6 2.3 4 6 2.7 5 7 3

WO₃ is an optional component of the optical glass of the present invention that raises the refractive index. WO₃ can be incorporated into the optical glass of the present invention within a range of 0 to 10 percent, having the effect of markedly raising the refractive index. Incorporating a quantity of WO₃ of greater than zero percent has the effects of stabilizing the glass more than rare earth components, lowering the liquidus temperature more than rare earth components, and raising the viscosity somewhat. The incorporation of a quantity of WO₃ exceeding 10 percent is undesirable in that it markedly increases dispersion (lowers νd). From such a perspective, the upper and lower limits of the WO₃ content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, from the perspective of lowering dispersion, the upper limit of Form C is set to a lower value than for the basic forms. From the perspective of raising the refractive index, the lower limit of Form D is set to a higher value than for the basic Forms.

TABLE 1-11 WO₃ (Cation %) Form Basic A B C D Upper limit 1 10 5 5 2 10 2 8 3 3 1 8 3 5 2 2 0.5 7 4 3 1 1 0 6 5 2 0 0 5.5 6 1 7 0 Lower limit 1 0 0 2 1 3 2 4 3 5 4

NbO_(2.5) is an optional component of the optical glass of the present invention that raises the refractive index. NbO_(2.5) can be incorporated into the optical glass of the present invention within a range of 0 to 8 percent, having the effect of markedly raising the refractive index. Incorporating a quantity of NbO_(2.5) of greater than zero percent has the effects of stabilizing the glass more than rare earth components, lowering the liquidus temperature more than rare earth components, and raising the viscosity somewhat. The incorporation of a quantity of NbO_(2.5) exceeding 8 percent is undesirable in that it markedly increases dispersion (lowers νd). From such a perspective, the upper and lower limits of the NbO_(2.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, from the perspectives of lowering dispersion and the liquidus temperature, the upper limits of Forms A, B, and C are set to a lower value than for the basic forms.

TABLE 1-12 NbO_(2.5) (Cation %) Form Basic A B C D Upper limit 1 8 3 2 2 5 2 5 2 1 1 3 3 3 1 0 0.5 2 4 2 0 0 1 5 1 0 6 0 Lower limit 1 0 0

TiO₂ is an optional component of the optical glass of the present invention that raises the refractive index. TiO₂ can be incorporated into the optical glass of the present invention within a range of 0 to 8 percent, having the effect of markedly raising the refractive index. Incorporating a quantity of TiO₂ of greater than zero percent has the effects of stabilizing the glass more than rare earth components, lowering the liquidus temperature more than rare earth components, and raising the viscosity somewhat. The incorporation of a quantity of TiO₂ exceeding 8 percent is undesirable in that it markedly increases dispersion (lowers νd). From such a perspective, the upper and lower limits of the TiO₂ content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, from the perspective of lowering dispersion, the upper limits of Forms A, B, and C are set to a lower value than for the basic forms.

TABLE 1-13 TiO₂ (Cation %) Form Basic A B C D Upper limit 1 8 3 5 2 5 2 5 2 3 1 3 3 3 1 2 0.5 2 4 2 0 1 0 1 5 1 0 0 6 0 Lower limit 1 0

AlO_(1.5) is an optional component of the optical glass of the present invention that forms a glass network. AlO_(1.5) can be incorporated into the optical glass of the present invention within a range of 0 to 10 percent, having the effect of somewhat enhancing glass stability. Incorporating a quantity of AlO_(1.5) of greater than zero percent has the effects of lowering dispersion, somewhat raising viscosity, enhancing chemical durability, and increasing mechanical strength. The incorporation of a quantity of AlO_(1.5) exceeding 10 percent is undesirable in that it lowers the refractive index and tends to raise the glass transition temperature and liquidus temperature. From such perspectives, the upper and lower limits of the AlO_(1.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, from the perspective of raising the refractive index, the upper limit of Form D is set to a lower value than for the basic forms.

TABLE 1-14 AlO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 10 3 5 10 3 2 7 2 3 7 2 3 5 1 2 5 1 4 3 0 1 3 0 5 2 0 2 6 1 1 7 0.5 0 8 0 Lower limit 1 0

GeO₂ is an optional component of the optical glass of the present invention that forms a glass network. GeO₂ can be incorporated into the optical glass of the present invention within a range of 0 to 5 percent, having the effect of somewhat increasing the stability of the glass. Incorporating a quantity of GeO₂ of greater than zero percent has the effects of somewhat increasing viscosity and raising the refractive index more than SiO₂. The incorporation of a quantity of GeO₂ exceeding 5 percent is undesirable in that it raises the cost of the glass. From such perspectives, the upper and lower limits of the GeO₂ content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. Although the use of GeO₂ is advantageous in terms of characteristics, it is quite expensive. Thus, the quantity of GeO₂ is desirably minimized to obtain an optical glass having desired characteristics.

TABLE 1-15 GeO₂ (Cation %) Form Basic A B C D Upper limit 1 5 5 5 5 5 2 3 3 3 3 3 3 2 2 2 2 2 4 1 1 1 1 1 5 0 0 0 0 0 Lower limit 1 0

GaO_(1.5) is an undesirable component that forms a glass network and can be incorporated to the extent that characteristics are maintained. GaO_(1.5) can be incorporated in the optical glass of the present invention in a range of 0 to 5 percent, having the effect of somewhat enhancing glass stability. The incorporation of a quantity of GaO_(1.5) of greater than zero percent has the effects of somewhat increasing viscosity and raising the refractive index more than SiO₂. The incorporation of a quantity of GaO_(1.5) exceeding 5 percent is undesirable in that it raises the cost of the glass. From such perspectives, the upper and lower limits of the GaO_(1.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention.

TABLE 1-16 GaO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 5 3 3 3 3 2 3 2 2 2 2 3 2 1 1 1 1 4 1 0 0 0 0 5 0 Lower limit 1 0

PO_(2.5) is an undesirable component that forms a glass network and can be incorporated to the extent that characteristics are maintained. PO_(2.5) can be incorporated in the optical glass of the present invention in a range of from 0 to 5 percent. The incorporation of PO_(2.5) in a quantity exceeding 5 percent is undesirable in that it compromises the stability of the glass. From such perspectives, the upper and lower limits of the PO_(2.5) content is suitably determined as indicated below for the basic forms and Forms A, B, C, and D of the present invention.

TABLE 1-17 PO_(2.5) (Cation %) Form Basic A B C D Upper limit 1 5 3 3 3 3 2 3 2 2 2 2 3 2 1 1 1 1 4 1 0 0 0 0 5 0 Lower limit 1 0

NaO_(0.5), KO_(0.5), RbO_(0.5), and CsO_(0.5) are modifying components that can be incorporated to the extent that characteristics are maintained. The optical glass of the present invention can contain a total quantity of NaO_(0.5), KO_(0.5), RbO_(0.5), and CsO_(0.5) falling within a range of 0 to 10 percent. A quantity exceeding 10 percent is undesirable in that the stability of the glass is lower than with LiO_(0.5), the refractive index of the glass is reduced more than by LiO_(0.5), and the glass transition temperature rises more than with LiO_(0.5). From such perspectives, the upper and lower limits of the total quantity of NaO_(0.5), KO_(0.5), RbO_(0.5), and CsO_(0.5) are suitably set as indicated below.

TABLE 1-18 (Cation %) Form Basic A B C D Upper limit 1 10 5 5 7 5 2 7 3 3 5 3 3 5 2 2 3 2 4 3 1 1 2 1 5 2 0 0 1 0 6 1 0 7 0 Lower limit 1 0

MgO, CaO, SrO, and BaO are modifying components that can be incorporated to the extent that characteristics are maintained. The optical glass of the present invention can contain a total quantity of MgO, CaO, SrO, and BaO falling within a range of 0 to 10 percent. A quantity exceeding 10 percent is undesirable in that the stability of the glass is lower than with ZnO, the refractive index of the glass is reduced more than by ZnO, the glass transition temperature rises more than with ZnO, and the liquidus temperature rises more than with ZnO. From such perspectives, the upper and lower limits of the total quantity of MgO, CaO, SrO, and BaO are suitably set as indicated below.

TABLE 1-19 (Cation %) Form Basic A B C D Upper limit 1 10 7 7 10 7 2 7 5 5 7 5 3 5 3 3 5 3 4 3 2 2 3 2 5 2 1 1 2 1 6 1 0 0 1 0 7 0 0 Lower limit 1 0

ScO_(1.5), InO_(1.5), and LuO_(1.5) are modifying components that can be incorporated to the extent that characteristics are maintained. The optical glass of the present invention can contain a total quantity of ScO_(1.5), InO_(1.5), and LuO_(1.5) falling within a range of 0 to 10 percent. A quantity exceeding 10 percent is undesirable in that the stability of the glass is lowered more than by LaO_(1.5) and GdO_(1.5), the liquidus temperature is raised more than by LaO_(1.5) and GdO_(1.5), and the refractive index of the glass is reduced more than by LaO_(1.5) and GdO_(1.5) From such perspectives, the upper and lower limits of the total quantity of ScO_(1.5), InO_(1.5), and LuO_(1.5) are suitably set as indicated below.

TABLE 1-20 (Cation %) Form Basic A B C D Upper limit 1 10 3 3 5 3 2 7 2 2 3 2 3 5 1 1 2 1 4 3 0 0 1 0 5 2 0 6 1 7 0 Lower limit 1 0

YbO_(1.5) is a modifying component the incorporation of which is to be avoided to the extent possible due to absorption in the infrared region. The optical glass of the present invention can contain 0 to 9 percent of YbO_(1.5). A YbO_(1.5) content in excess of 9 percent is undesirable in that the stability of the glass is lowered more than by LaO_(1.5) and GdO_(1.5), the liquidus temperature is raised more than by LaO_(1.5) and GdO_(1.5), the refractive index of the glass is lowered more than by LaO_(1.5) and GdO_(1.5), and there is absorption in the infrared region. From such perspectives, the upper and lower limits of the YbO_(1.5) content are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. In particular, from the above-mentioned perspectives of cost, liquidus temperature, and infrared absorption, the upper limit of Form A is set lower than in the basic forms.

TABLE 1-21 YbO_(1.5) (Cation %) Form Basic A B C D Upper limit 1 9 0.5 3 3 3 2 5 0.4 2 2 2 3 3 0.2 1 1 1 4 1 0.1 0.5 0.5 0.5 5 0.4 0.0 0.4 0.4 0.4 6 0.2 0.2 0.2 0.2 7 0.1 0.1 0.1 0.1 8 0 0.0 0.0 0.0 Lower limit 1 0 0.1 2 0.2 3 0.4

BO_(1.5)×F/O is an indicator of suppression of variation in characteristics due to volatilization. BO_(1.5) is denoted as a cationic percentage and F/O is the molar ratio of F and O. BO_(1.5)×F/O falls within a range of 0.05 to 10.00. Volatilization of the glass is suppressed by reducing boron and fluorine. Accordingly, boron and fluorine compounds, and components produced by the reaction of such compounds with oxygen and water in the atmosphere, are thought to be the volatile components. Qualitatively, the quantity of such volatile components is proportional to the product of the boron concentration in the glass and the fluorine concentration in the glass. The smaller this value is, the lower the potential amount of volatilization from the glass. From such perspectives, the upper and lower limits of BO_(1.5)×F/O are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. Further, the value of BO_(1.5)×F/O can be suitably adjusted by adjusting the quantities of each of the components of BO_(1.5), F, and O incorporated into the optical glass. In particular, from the perspective of low dispersion, the upper limit for Form C is set higher than for the basic forms, and from the perspective of a high refractive index, the upper limit for Form D is set lower than for the basic forms. From the perspective of a high refractive index, the lower limit in Form B is set higher than for the basic forms.

The following components can be added to adjust the quantity of bubbles in the glass.

CeO₂ can be added within a range of 0 to 2 percent to the extent that the effect of the present invention is not spoiled. The quantity of CeO₂ added is desirably 0 to 1 percent, preferably falling within a range of 0 to 0.5 percent, more preferably 0 to 0.3 percent, still more preferably 0 to 0.1 percent, and yet more preferably, none is incorporated at all.

SnO₂ can be added within a range of 0 to 2 percent to the extent that the effect of the present invention is not spoiled. The quantity of SnO₂ added is desirably 0 to 1 percent, preferably 0 to 0.5 percent, more preferably 0 to 0.3 percent, still more preferably 0 to 0.1 percent, and yet more preferably, none is incorporated at all.

SbO_(1.5) can be added within a range of 0 to 1 percent to the extent that the effect of the present invention is not spoiled. The quantity of SbO_(1.5) added is desirably 0 to 0.5 percent, preferably falling within a range of 0 to 0.1 percent, more preferably falling within a range of 0 to 0.08 percent, still more preferably falling within a range of 0 to 0.05 percent. When SbO_(1.5) is added, 0.01 to 0.02 percent is desirably incorporated.

To achieve the object of the present invention, the total quantity of BO_(1.5), SiO₂, GeO₂, AlO_(1.5), LiO_(0.5), NaO_(0.5), KO_(0.5), MgO, CaO, SrO, BaO, ZnO, LaO_(1.5), GdO_(1.5), YO_(1.5), TiO₂, ZrO₂, TaO_(2.5), NbO_(2.5), WO₃, YbO_(1.5), CeO₂, SnO₂, and SbO_(1.5) is desirably 95 percent or greater, preferably 96 percent or greater, more preferably 97 percent or greater, still more preferably 98 percent or greater, still more preferably 99 percent or greater, and still more preferably 99.5 percent or greater, with 100 percent being optimal.

To achieve the object of the present invention, the total quantity of BO_(1.5), SiO₂, GeO₂, AlO_(1.5), LiO_(0.5), NaO_(0.5), KO_(0.5), MgO, CaO, SrO, BaO, ZnO, LaO_(1.5), GdO_(1.5), YO_(1.5), TiO₂, ZrO₂, TaO_(2.5), NbO_(2.5), WO₃, YbO_(1.5), CeO₂, SnO₂, SbO_(1.5) GaO_(1.5), PO_(2.5), ScO_(1.5), InO_(1.5), and LuO_(1.5) is desirably 98 percent or greater, preferably 99 percent or greater, more preferably 99.5 percent or greater, still more preferably 99.8 percent or greater, and still more preferably 99.9 percent or greater, with 100 percent being optimal.

TABLE 1-22 BO_(1.5) × F/O Form Basic A B C D Upper limit 1 10.00 8 10 15 5.0 2 8.00 6 9 12 4.5 3 6.00 5 8 10 4.0 4 5.00 4.75 7.5 8 3.5 5 4.75 4.5 7.25 6 3.0 6 4.50 4.25 7 5 2.5 7 4.25 4 6.75 2.0 8 4.00 6.5 Lower limit 1 0.50 0.5 0.5 2 0 2 1.00 1 2 2.5 0.5 3 2.00 2 3.5 3 1.0 4 2.50 2.5 4.5 3.5 1.5 5 3.00 5 4 6 5.5 7 6

Excess B—O—B bonds: D (=B_(B)—(B₀—B_(B))) is an indicator of the suppression of variation in characteristics due to volatilization, and falls within a range of −60 to +60. Among glass components, the volatilization of boron in particular affects the glass composition. The initial reaction marking the start of volatilization is thought to consist of the recombination of boron-oxygen bonds in the glass by fluorine ions. At that time, the number density of B—O bonds contributing to the reaction in the glass is though to be given by subtracting the number density of bridging bonds between cations other than boron within the glass from the number density of boron bonds in the glass, that is, is thought to be qualitatively proportional to the number density of excess boron bonds. Accordingly, the smaller this value becomes, the more the reaction between boron and fluorine is suppressed, and the more the volatilization rate is reduced. A particularly marked effect is achieved by reducing this value to below zero. Accordingly, the lower the fluorine content and the fewer the excess B—O—B bonds that are present, the greater the suppression effect on volatilization. When this value is lower than 0, that is, when the quantity of oxygen bonded to boron is smaller than the quantity of oxygen bonded to other components, the effect is particularly pronounced. It is not just B×F, but rather D×F, that is desirably low.

FIG. 10 shows the change in weight reduction (volatilization) due to change in the F content in optical glasses of Forms A to D of the present invention, with the F content denoted by the horizontal axis and the change in weight denoted by the vertical axis. From this graph, it will be understood that the reduction in weight tends to increase as the F content increases. FIG. 11 shows the relation between the value of D and the reduction in weight for glasses with F contents of 7 to 9 percent, with the value of D denoted by the horizontal axis and the reduction in weight denoted by the vertical axis. FIG. 12 shows the relation between the value of D and the reduction in weight in optical glasses with F contents of 14 to 16 percent, with the value of D denoted by the horizontal axis and the reduction in weight denoted by the vertical axis. FIG. 14 combines FIGS. 12 and 13.

The reduction in weight is caused by volatilization from the glass melt, and is thus a quantitative indicator of volatilization. F sometimes volatizes alone, and sometimes cleaves the B—O—B bond formed in the glass network and volatizes in the form of molecules in which fluorine is bonded to other glass components, such as BF₃ and BF₂ ⁺. It is primarily the latter which cause serious volatilization striae and changes in characteristics over time during molding of the glass.

In the volatilization produced by fluorine, fluorine compounds that do not contain boron, such as SiF₄, are also present. However, due to differences in their vapor pressure, most volatizing components can be thought of as being comprised of fluorine and boron (along with glass and atmospheric oxygen and hydrogen).

A comparison of the B—O—B bond with the B—O—X bond (where X denotes a cation other than B) in the glass network reveals that the probability of forming a volatile component in the B—O—B bond is greater when subjected to a replacement reaction of oxygen with fluorine caused by fluorine.

Since large numbers of B—O—B bonds present in glasses with high values of D, glasses with high values of D tend to undergo greater volatilization of components within the glass. However, despite the B network tending to cleave readily, volatile compounds tend not to be produced so long as the content of F is low.

Even when the F content is high, in a case when the value of D is low and cleavage of the B network tends not to occur, volatile compounds tend not to be produced.

Thus, the F content and the value of D are factors that greatly affect the level of volatilization.

Other than B, rare earth components, typified by La, are cations X that have a strong ability to retain both B and F within the glass without volatilization. Accordingly, as set forth further below, following the F content and the value of D, the value obtained by multiplying the BO_(1.5) content by the F content and then dividing the product by the LaO_(1.5) content—namely, (BO_(1.5)×F)/LaO_(1.5)—is conceivable as an indicator affecting the level of volatilization. Reducing (BO_(1.5)×F)/LaO_(1.5) further suppresses volatilization.

From such perspectives, the upper and lower limits of excess B—O—B bonds: D are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The value of excess B—O—B bonds: D can be suitably adjusted by adjusting the quantities of BO_(1.5), F, and other components that are incorporated into the optical glass.

(Computation Methods)

-   B_(B)=(quantity of BO_(1.5))×1.5 -   B_(O)=total quantity of oxygen -   D=B_(B)—(B_(O)—B_(B)) -   The quantity of BO_(1.5) is the value when the content of BO_(1.5)     is denoted as a cationic percentage. -   B_(O) is the relative value of the quantity of O when the total     quantity of all cations is adopted as 100.

TABLE 1-23 D (= B_(B) − (B_(O) − B_(B))) Form Basic A B C D Upper limit 1 60 20 60 0 2 40 10 50 −10 3 20 5 20 40 −20 4 10 0 10 30 −25 5 5 −5 0 20 −30 6 0 −10 −10 15 7 −5 −15 8 −10 −20 Lower limit 1 −60 −60 −60 −20 −45 2 −40 −40 −50 −15 −50 3 −30 −30 −40 −10 −55 4 −25 −25 −35 −5 −60 5 −20 −20 −30 0

-   BO_(1.5)×F/LaO_(1.5)

In the glass of the present invention, in which LaO_(1.5) is an essential component, it is desirable to adjust BO_(1.5)×F/LaO_(1.5), which is an indicator obtained by multiplying BO_(1.5) by F and dividing the product by LaO_(1.5), to further adjust the level of volatilization while maintaining a high refractive index and low dispersion. The incorporation of LaO_(1.5), which is capable of bonding both oxygen and fluorine, reduces the quantity of fluorine that actually contributes to volatilization. That is, the smaller BO_(1.5)×F/LaO_(1.5) is made, the less volatilization occurs. The value of BO_(1.5)×F/LaO_(1.5) can be suitably set by adjusting BO_(1.5), F, LaO_(1.5), and other components.

TABLE 1-24 BO_(1.5) × F/LaO_(1.5) Form A B C D Upper limit 1 45 42 48 18 2 42 40 45 15 3 40 38 42 13 4 38 35 40 11 5 35 33 38 6 30 30 35 7 25 28 30 8 25 Lower limit 1 5 3 8 1 2 8 5 10 2 3 10 8 14 3 4 14 10 18 5 5 18 14 22 8 6 18

SiO₂/BO_(1.5), the ratio of the SiO₂ content to the BO_(1.5) content, is an indicator of suppression of molding striae due to increased viscosity that desirably falls within a range of 0.10 to 0.40. Among glass network-forming components, the use of a large amount of viscosity-increasing SiO₂ raises the viscosity during molding and suppresses deterioration of the internal quality of the glass due to the generation of striae. From such a perspective, the upper and lower limits of SiO₂/BO_(1.5) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The value of SiO₂/BO_(1.5) can be suitably set by adjusting the quantities of the components SiO₂ and BO_(1.5) that are incorporated into the optical glass.

TABLE 1-25 SiO₂/BO_(1.5) Form Basic A B C D Upper limit 1 0.40 0.55 0.4 0.4 0.20 2 0.36 0.50 0.36 0.36 0.18 3 0.34 0.40 0.34 0.34 0.16 4 0.32 0.36 0.32 0.32 0.14 5 0.30 0.34 0.3 0.3 0.13 6 0.28 0.32 0.28 0.28 7 0.30 0.27 0.25 8 0.28 Lower limit 1 0.10 0.1 0 0.05 0.03 2 0.15 0.15 0.1 0.1 0.05 3 0.17 0.17 0.15 0.15 0.07 4 0.19 0.19 0.17 0.17 0.09 5 0.21 0.21 0.19 0.19 0.1 6 0.22 0.22 0.21 0.21 0.11 7 0.23 0.22 8 0.23

LaO_(1.5)/ΣRe (ΣRe=total quantity of rare earth components), the ratio of the content of LaO_(1.5) to the total content of rare earth components, is an indicator of suppression of the formation of striae due to lowering of the liquidus temperature that desirably falls within a range of 0.30 to 0.80. Optimizing the ratio of rare earth components that raise the melting point of the glass lowers the melting point through an eutectic effect. This effect lowers the minimum temperature at which crystals precipitate out into the glass melt, or the liquidus temperature, thereby increasing the maximum viscosity at which the glass melt will flow out continuously. From such perspectives, the upper and lower limits of LaO_(1.5)/ΣRe are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The value of LaO_(1.5)/ΣRe can be suitably set by adjusting the quantities of LaO_(1.5) and various rare earth components that are incorporated into the optical glass.

TABLE 1-26 LaO_(1.5)/ΣRe Form Basic A B C D 1 0.80 0.75 0.8 0.9 0.9 2 0.70 0.7 0.75 0.8 0.85 3 0.60 0.6 0.7 0.7 0.8 4 0.57 0.57 0.67 0.6 0.77 5 0.55 0.55 0.65 0.57 0.75 6 0.53 0.53 0.63 0.55 7 0.52 0.52 0.62 Lower limit 1 0.30 0.3 0.3 0.3 0.5 2 0.40 0.4 0.4 0.4 0.55 3 0.45 0.45 0.5 0.45 0.6 4 0.48 0.48 0.55 0.48 0.65 5 0.49 0.49 0.58 0.7 6 0.59

2LiO_(0.5)+ZnO+(F/2)—the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content—is an indicator of enhanced precision press moldability due to a decrease in the glass transition temperature Tg that desirably falls within a range of 20 to 60. The effect of lowering the Tg of a glass by incorporating Li, Zn, and F is 2 for Li, 1 for Zn, and 0.5 for F per atomic percent. Thus, the sum thereof, 2LiO_(0.5)+ZnO+(F/2), is approximately linked to the glass transition temperature Tg of the glass. The magnitude of this indicator is an indicator of the ease of press molding. LiO_(0.5) and ZnO are values denoted as cationic percentages. F is the relative value of the quantity of F when the total quantity of all cations is adopted as 100. From such a perspective, the upper and lower limits of 2LiO_(0.5)+ZnO+(F/2) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The value of 2LiO_(0.5)+ZnO+(F/2) can be suitably set by adjusting the quantities of the components LiO_(0.5), ZnO, and F that are incorporated into the optical glass.

FIG. 5 shows how the Tg changes with change in 2LiO_(0.5)+ZnO+(F/2), with the value of 2LiO_(0.5)+ZnO+(F/2) be denoted by the horizontal axis and the value of the glass transition temperature Tg being denoted by the vertical axis. As will be clear from FIG. 5, the Tg can be lowered by increasing 2LiO_(0.5)+ZnO+(F/2) and the Tg can be raised by decreasing 2LiO_(0.5)+ZnO+(F/2).

TABLE 1-27 2LiO_(0.5) + ZnO + (F/2) Form Basic A B C D Upper limit 1 60 45 45 40 2 50 40 45 40 37 3 45 37 40 37 35 4 40 35 37 35 32 5 37 34 35 34 6 35 34 7 34 8 Lower limit 1 20 25 20 25 20 2 25 28 23 28 22 3 28 29 26 29 24 4 29 30 28 30 25 5 30 31 29 31 26 6 31 30 28

2LiO_(0.5)/(2LiO_(0.5)+ZnO)—the value of twice the LiO_(0.5) content divided by the combined sum of twice the LiO_(0.5) content and the ZnO content—contributes to enhancing glass characteristics (by the effects of suppressing a drop in νd and lowering the specific gravity) by adjusting Tg-lowering components and desirably falls within a range of 0.10 to 1.00. Among components that lower the glass transition temperature Tg of the glass, Li lowers νd less than Zn and has the effect of increasing the refractive index nd relative to the specific gravity of the glass obtained. It is thus employed to obtain a glass with a high refractive index, low dispersion, a low specific gravity, and a high kinematic viscosity. From such perspectives, the upper and lower limits of 2LiO_(0.5)/(2LiO_(0.5)+ZnO) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. Further, the value of 2LiO_(0.5)/(2LiO_(0.5)+ZnO) can be suitably set by adjusting the quantities of the components LiO_(0.5) and ZnO that are incorporated into the optical glass.

TABLE 1-28 2LiO_(0.5)/(2LiO_(0.5) + ZnO) Form Basic A B C D Upper limit 1 1.00 1.0 1.0 1.0 2 0.90 0.9 0.9 0.9 3 0.85 0.85 0.8 0.8 4 0.7 0.7 5 0.6 0.6 6 0.5 0.5 7 0.45 0.4 8 0.4 Lower limit 1 0.10 0.1 0 0.75 0 2 0.30 0.3 0.1 0.8 0.05 3 0.40 0.4 0.15 0.85 0.1 4 0.50 0.5 0.2 0.9 0.15 5 0.60 0.6 0.25 0.95 0.2 6 0.65 0.65 0.3 1 0.25 7 0.70 0.7 8 0.75 0.75

ZnO+3ZrO₂+5TaO_(2.5), the combined sum of the Zn content, three times the ZrO₂ content, and 5 five times the TaO_(2.5) content, is an indicator of the maintenance of low dispersion by controlling the quantities of high dispersion components that desirably falls within a range of 0 to 60.0. The effect of lowering the νd of a glass by incorporating Zn, Zr, and Ta into the glass is 1 for Zn, 3 for Zr, and 5 for Ta per atomic percent. Accordingly, the sum of ZnO+3ZrO₂+5 TaO_(2.5) is approximately linked to the νd of the glass, so the magnitude of this indicator is an indicator of how low the dispersion is. It is also possible to include indicators such as W, Nb, and Ti that lower dispersion even more than Ta. However, as set forth above, the incorporation of large quantities of W, Nb, and Ti is undesirable. From such perspectives, the upper and lower limits of ZnO+3ZrO₂+5TaO_(2.5) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The value of ZnO+3ZrO₂+5TaO_(2.5) can be suitably set by adjusting the quantities of components ZnO, ZrO₂, and TaO_(2.5) that are incorporated into the optical glass.

TABLE 1-29 ZnO + 3ZrO₂ + 5TaO_(2.5) Form Basic A B C D Upper limit 1 60.0 40 60 12 80 2 40.0 30 50 8 70 3 30.0 25 46 5 65 4 25.0 22 43 3 60 5 22.0 21 40 2 55 6 21.0 20.5 39 1 7 20.5 20 38.5 0 8 20.0 38 Lower limit 1 0.0 0 30 2 1.0 1 10 35 3 6.0 6 20 40 4 10.0 10 30 45 5 14.0 14 34 50 6 16.0 16 36 7 18.0 18 36.5

The glass transition temperature Tg (° C.) is an indicator of the degree of difficulty of precision press molding that desirably falls within a range of 200 to 590° C. A low glass transition temperature permits a low press molding temperature. It can be measured with a differential thermal analyzer DSC. From such perspectives, the upper and lower limits of the glass transition temperature Tg (° C.) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The glass transition temperature Tg (° C.) can be suitably set primarily by adjusting the quantities of the components LiO_(0.5), ZnO, and F that are incorporated into the optical glass. Further, microadjustment is possible by adjusting the quantities of the components SiO₂ and TaO_(2.5) that are incorporated into the optical glass.

TABLE 2-1 Glass transition temperature Tg (° C.) Form Basic A B C D Upper limit 1 590 580 590 600 590 2 580 575 580 590 580 3 575 570 575 580 570 4 570 565 570 575 565 5 565 560 565 570 560 6 560 555 560 565 7 555 550 555 560 8 550 540 550 9 540 540 Lower limit 1 200 200 200 200 200 2 300 300 300 300 300 3 400 400 400 400 400 4 500 5 530

The liquidus temperature LT (° C.) is an indicator of glass stability that desirably falls within a range of 500 to 1,140° C. The minimum temperature at which no precipitation of crystals is observed when a glass melt is maintained in a furnace is defined as the liquidus temperature (LT). The liquidus temperature LT (° C.) (when the furnace temperature is set in increments of 10° C.) can be measured by placing 30 cc of glass in a platinum crucible, melting the glass for 10 to 20 minutes in a furnace at 1,200 to 1,250° C., homogenizing the melt by stirring, cooling the obtained glass in the crucible to below the glass transition temperature Tg, placing a platinum cover on the crucible, and maintaining the crucible in a furnace set to 1,050 to 1,140° C. for two hours to observe the crystals precipitating on the surface of the glass, within the glass, and at the interface of the glass and the inner wall of the crucible. From such a perspective, the upper and lower limits of the liquidus temperature LT (° C.) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. The liquidus temperature LC (° C.) can be suitably set by adjusting the quantities of components that are incorporated into the optical glass, such as BO_(1.5) and SiO₂, to enhance the thermal stability of the glass. The liquidus temperature can be lowered by improving the thermal stability of the glass. Within the composition ranges of the various forms, the liquidus temperature of the glass tends to decrease with improvement in thermal stability.

TABLE 2-2 Liquidus temperature LT (° C.) Form Basic A B C D Upper limit 1 1150 1120 1140 1120 1140 2 1120 1100 1120 1100 1120 3 1100 1090 1100 1090 1100 4 1090 1080 1090 1080 1090 5 1080 1070 1080 1070 1080 6 1070 1060 1070 1060 1070 7 1060 1050 1060 1050 1060 8 1050 1050 1050 Lower limit 1 500 500 500 500 500 2 600 600 600 600 600 3 700 700 700 700 4 800

λ(lambda) 80, λ70, and λ5 (nm) are wavelengths (nm) at which 80 percent, 70 percent, and 5 percent transmittance to the exterior, respectively, are exhibited by a sample 10 mm in thickness. From the perspective of inhibiting specific wavelength dependence in a particular glass, at a minimum, the external transmittance from λ80 to 1,550 nm is desirably not lowered to 25 percent or below, preferably 50 percent or below, more preferably 70 percent or below, still more preferably 75 percent or below, and yet more preferably, 80 percent or below. The external transmittance at λ80, λ70, λ5 (nm), and the wavelength range of λ80 to 1,555 nm, can be suitably adjusted by adjusting the quantities of various components with different ultraviolet absorption edges that are incorporated into an optical glass; limiting the quantities of impurities in the form of such color-producing elements as Fe, Cr, Mn, Co, and Ni; incorporating various oxidizing gas components (NO₃ and the like); and reducing gas components (NH₄ and the like). Thus, it is desirable not to incorporate any Fe, Cr, Mn, Co, or Ni, and to limit the quantities of impurities in the form of Fe, Cr, Mn, Co, and Ni that are mixed in.

TABLE 2-3 λ80 Form Basic A B C D Upper limit 1 450 450 450 550 2 400 400 400 500 3 380 380 380 470 4 450 Lower limit 1 300 300 300 350 2 330 330 330 370 3 350 350 350 400

TABLE 2-4 λ70 Form Basic A B C D Upper limit 1 450 450 450 500 2 400 400 400 450 3 370 370 370 400 4 370 Lower limit 1 250 250 250 300 2 300 300 300 330 3 320 320 320 350 350

TABLE 2-5 λ5 Form Basic A B C D Upper limit 1 350 350 350 450 2 320 320 320 400 3 290 290 290 350 Lower limit 1 200 200 200 250 2 250 250 250 300 3 270 270 270 320 330

The value (kinematic viscosity) (10⁻⁴ m²/s=St) obtained by dividing the viscosity at the liquidus temperature by the density at room temperature is an indicator of the moldability of the glass melt that desirably falls within a range of 0.20 to 1,000 (St). It is obtained by dividing the viscosity at the liquidus temperature, η(eta)_(LT)(P) by the specific gravity (g/cc). It corresponds to the flow rate as the glass flows out of a pipe or the like under its own weight, and serves as a yardstick of the ease with which the glass melt can be molded. Considering the specific gravity of the glass facilitates comparison of glasses with different specific gravities and their flow rates. From such perspectives, the upper and lower limits of the value obtained by dividing the viscosity at the liquidus temperature by the density at room temperature are suitably set as indicated below. The value of the kinematic viscosity can be suitably set by adjusting the quantities of the various components that are incorporated into the optical glass, such as BO_(1.5), which has the effect of improving the thermal stability of the glass, and SiO₂, which has the effects of increasing the viscosity of the glass while similarly improving the thermal stability of the glass. Improving the thermal stability of the glass lowers the liquidus temperature and increases the viscosity at the liquidus temperature. As compared to rare earth components, BO_(1.5) and SiO₂ have the additional function of reducing the specific gravity of the glass. Thus, increasing the quantities of BO_(1.5) and SiO₂ that are incorporated increases the kinematic viscosity.

TABLE 2-6 Kinematic viscosity at liquidus temperature (St) Form Basic A B C D Upper limit 1 1000 30 30 30 30 2 300 10 10 10 10 3 100 3 3 3 3 4 30 1 1 1 1 5 10 6 3 7 1 8 Lower limit 1 0.2 0.4 0.4 0.4 0.2 2 0.4 0.5 0.5 0.5 0.4 3 0.5 0.55 0.55 0.55 0.5 4 0.55 0.6 0.6 0.6 0.55 5 0.6 0.65 0.65 0.65 0.6 6 0.65 0.7 0.7 0.7 0.65 7 0.7 0.75 0.75 0.75 0.7 8 0.75 0.8 0.8 0.8

The quantity of glass flowing per unit time, that is, the flow rate, is proportional to the density of the glass and is inversely proportional to the viscosity of the glass. Accordingly, the inverse of the value of the kinematic viscosity is proportional to the flow rate. Thus, the inverse of the kinematic viscosity is an indicator of the flow rate.

FIG. 6 shows the change in the inverse of the kinematic viscosity and the change in the viscosity relative to the change in temperature in an example of the optical glass of the present invention, with temperature being denoted by the horizontal axis, viscosity of the glass being denoted by the left vertical axis, and the inverse of the kinematic viscosity, that is, an indicator of the flow rate, being denoted by the right vertical axis.

In FIG. 6, it will be understood that the amount of increase in the indicator of the flow rate (the inverse of the kinematic viscosity) due to a decrease of 1 dPa·s in viscosity was small in the region of relatively high viscosity (such as a viscosity of equal to or greater than 10 dPa·s, that is, at equal to or below 940° C.).

However, in the region of relatively low viscosity (such as a viscosity of 5 dPa·s, 3 dPa·s, or the like), the amount of increase in the indicator of the flow rate (the inverse of the kinematic viscosity) due to a decrease of 1 dPa·s in viscosity increased. Thus, even for a difference in the viscosity value of just 0.5 or 1.0 dPa·s, the corresponding change in the flow rate was substantial, greatly affecting glass moldability. In glasses with a liquidus temperature exceeding 1,050° C., or of 1,100° C. or higher, that is, in glasses in which the viscosity at the liquidus temperature is low like above, the reason a change of just 10° C. in liquidus temperature greatly affects moldability is due to such facts.

TABLE 2-7 Viscosity at liquidus temperature (dPa · s) Form Basic A B C D Upper limit 1 5000 5000 5000 5000 5000 2 1500 1500 1500 1500 1500 3 500 500 500 500 500 4 150 150 150 150 150 5 50 50 50 50 50 6 15 15 15 15 15 7 5 5 5 5 5 Lower limit 1 1 1 1 1 1 2 2 2 2 2 2 3 2.5 2.5 2.5 2.5 2.5 4 2.75 2.75 2.75 2.75 2.75 5 3 3 3 3 3 6 3.25 3.25 3.25 3.25 3.25 7 3.5 3.5 3.5 3.5 3.5 8 3.75 3.75 3.75 3.75 3.75

When the viscosity is low at the liquidus temperature, problems tend to occur in that striae tend to form, it becomes difficult to control the flow of glass, and the like. When the viscosity is high at the liquidus temperature, the glass flow rate drops, productivity decreases, or the holding time of the glass melt in the melting furnace becomes excessively long, causing variation in various characteristics.

The optical glass of the present invention is suited to lenses with large diameters that are employed in single-lens reflex cameras and the like. Thus, a low specific gravity is desirable. From such perspectives, the upper and lower limits of the specific gravity are suitably set as indicated below. The specific gravity can be measured by Archimedes' method. The specific gravity can be suitably set by adjusting the quantities of various components of relatively low molecular weight, such as BO_(1.5) and SiO₂, and rare earth components of relatively high molecular weight that are incorporated into the optical glass.

TABLE 2-8 Specific gravity Form Basic A B C D Upper limit 1 6.00 6 5.30 6 2 5.50 5.5 5.8 5.10 5.9 3 5.30 5.30 5.6 5.00 5.8 4 5.10 5.10 5.5 4.90 5.7 5 5.00 5.00 5.4 4.80 5.6 6 4.90 4.90 5.3 4.70 7 4.80 4.80 5.2 4.60 8 4.70 4.70 5.1 4.5 Lower limit 1 2.0 2.0 2 2.0 2 3.0 3.0 3 3.0 4 3 4.0 4.0 4 3.5 4.5 4 4.5 4.5 4.5 3.8 5 5 4.6 4.6 5 4.0 The degrees of the refractive index and dispersion: nd−(2.25−0.01×νd)

The optical glass of the present invention is of high refractive index and low dispersion. The degrees thereof are desirably a refractive index that lies above and a dispersion that lies below a line drawn connecting (nd, νd)=(1.80, 45) and (1.75, 50) in a plot of nd−νd in which nd is denoted by the horizontal axis and νd is denoted by the vertical axis. Further, a certain quantity of glass forming components is required to maintain moldability of the glass. Thus, glass compositions in which the refractive index is so high and the dispersion so low that glass moldability is lost are undesirable. From such perspectives, the upper and lower limits of nd−(2.25−0.01×νd) are suitably set as indicated below for the basic forms and Forms A, B, C, and D of the present invention. nd−(2.25−0.01×νd) can be computed from the refractive index nd and the Abbé number νd. The value of nd−(2.25−0.01×νd) increases with the quantity of rare earth components and decreases with increases in the quantities of BO_(1.5), ZnO, ZrO₂, TaO_(2.5), and the like, and can thus be suitably set by adjusting the quantities of these components that are incorporated into the optical glass.

TABLE 2-9 nd − (2.25 − 0.01 × νd) Form Basic A B C D Upper limit 1 0.050 0.040 0.040 0.040 0.040 2 0.040 0.030 0.030 0.030 0.030 3 0.030 0.025 0.025 0.025 0.025 4 0.025 0.022 0.022 0.022 0.022 5 0.022 0.020 0.021 0.019 6 0.020 0.020 0.016 7 0.014 8 0.013 Lower limit 1 −0.010 −0.010 −0.010 −0.010 2 −0.005 −0.005 −0.005 −0.005 3 0.000 0.000 0.000 0.000 0.000 4 0.005 0.005 0.005 0.005 0.002 5 0.008 0.008 0.010 0.008 0.004 6 0.010 0.010 0.012 0.010 0.006 7 0.012 0.012 0.014 0.012 0.008 8 0.013 0.013 0.016 0.013 0.009 9 0.014 0.014 0.017 0.014 10 0.015 0.018 0.015 The shift away from the stoichiometric ratio (oxygen+fluorine/2)/cations (molar ratio)

Fluorine ions are primarily introduced into the glass by replacing a certain cationic percentage of oxides with fluorides. In this process, to inhibit coloration and the like due to lattice defects typified by oxygen defects and cation defects in the glass, the cations and anions in the glass are desirably introduced in a form that maintains charge neutrality in the glass. Accordingly, components employed to manufacture the glass are desirably incorporated so as to maintain the stoichiometric ratio of the glass by replacing one oxygen atom with two fluorine atoms, and so forth. From such perspectives, the upper and lower limits of the shift in the stoichiometric ratio are suitably set as indicated below. It can be calculated from the molar percentage of oxygen ions, the molar percentage of fluorine ions, and the total of cationic percentage. The shift away from the stoichiometric ratio can be suitably set by incorporating various oxidizing gas components (such as NO₃), reducing gas components (such as NH₄), and the like when transition metal oxides (such as ZrO₂, TaO_(2.5), TiO₂, NbO_(2.5), and WO₃) are being incorporated. Almost no shift away from the stoichiometric ratio occurs when incorporating typical metal oxides.

TABLE 2-10 Desirable upper limit Stoichiometric ratio ± 0.1 Preferable upper limit Stoichiometric ratio ± 0.05 More preferable upper limit Stoichiometric ratio ± 0.01 Still more preferable upper limit Substantially equal to stoichiometric ratio

The molar percentage of oxygen ions defines the unit oxygen number (for example: the oxygen number of BO_(1.5) is 1.5) per cation contained in the chemical formula of each component for introducing cations. The oxygen ion/cation (molar ratio) is the cationic percentages in the glass multiplied by the sum of the unit oxygen, divided by the sum of the cationic percentages, expressed as a percentage. Similarly, the molar percentage of fluorine ions is the sum of the ion percentages of the fluorine incorporated into the glass, divided by the sum of the cationic percentages, expressed as a percentage. From such perspectives, the upper and lower limits are suitably set as indicated below. They can be calculated from the molar percentage of oxygen ions and from the cationic percentages. The oxygen ion/cation (molar ratio) can be suitably set by incorporating various oxidizing gas components (such as NO₃), reducing gas components (such as NH₄), and the like when incorporating glass components in the form of transition metal oxides (such as ZrO₂, TaO_(2.5), TiO₂, NbO_(2.5), and WO₃). The oxygen ion/cation (molar ratio) increases with the quantity of cations of high valence that are included among the glass components, but tends to decrease as the F/O ratio increases.

TABLE 2-11 Oxygen ion/cation (molar ratio) Form Basic A B C D Upper limit 1 2.0 2.0 2.0 2.0 2.0 2 1.8 1.8 1.8 1.8 1.8 3 1.7 1.7 1.7 1.7 1.7 4 1.6 1.6 1.6 1.6 1.6 5 1.5 1.5 1.5 1.5 1.5 Lower limit 1 0.5 0.5 0.5 0.5 0.5 2 0.8 0.8 0.8 0.8 0.8 3 1.0 1.0 1.0 1.0 1.0 4 1.1 1.1 1.1 1.1 1.1 5 1.2 1.2 1.2 1.2 1.2 6 1.3 1.3 1.3 1.3 1.3 The change in nd over time: Δ(DELTA)nd(/hr)

The change in the refractive index over time is a measurement value obtained from various measured values of the refractive index and a sampling time interval by sampling the glass melt during melting, measuring the refractive index nd of the glass sample, allowing a prescribed interval of time to elapse following the sampling, sampling the glass melt again, measuring the refractive index nd of the glass sample, and repeating this operation. The change in the refractive index over time can be reduced by reducing excess B—O—B bonds and reducing BO_(1.5)×F/O, which is the quantity of volatile components. From such perspectives, the upper and lower limits of the change in the refractive index over time, Δnd(/hr), are suitably set as indicated below. The change in nd over time is caused by the change, albeit slight, in the glass composition due to volatilization from the glass melt Inhibiting this volatilization can reduce the change in refractive index nd over time. The level of volatilization can be adjusted by adjusting the quantities of various components incorporated into the optical glass, including BO_(1.5) and F. The quantities of BO_(1.5) and F that are incorporated into the optical glass have the greatest effect on the level of volatilization.

TABLE 2-12 Form Basic A B C D Upper limit 1 0.00200 0.00200 0.00200 0.00200 0.00200 2 0.00100 0.00100 0.00100 0.00100 0.00100 3 0.00080 0.00080 0.00080 0.00080 0.00080 4 0.00060 0.00060 0.00060 0.00060 0.00060 5 0.00040 0.00040 0.00040 0.00040 0.00040 6 0.00030 0.00030 0.00030 0.00030 0.00030 7 0.00020 0.00020 0.00020 0.00020 0.00020 8 0.00010 0.00010 0.00010 0.00010 0.00010 9 0.00005 0.00005 0.00005 0.00005 0.00005 Lower limit 1 0.00000 0.00000 0.00000 0.00000 0.00000 The change in Tg over time: Δ(DELTA)Tg (° C./hr)

The change in Tg over time is a measurement value obtained from various measured values of the glass transition temperature Tg and a sampling time interval by sampling the glass melt during melting, measuring the glass transition temperature Tg of the glass sample, allowing a prescribed interval of time to elapse following the sampling, sampling the glass melt again, measuring the glass transition temperature Tg of the glass sample, and repeating this operation. The change in Tg over time can be reduced by reducing excess B—O—B bonds and reducing BO_(1.5)×F/O, which is the quantity of volatile components. From such perspectives, the upper and lower limits of the change in Tg over time, ΔTg (° C./hr), are suitably set as indicated below. The change in Tg over time is caused by the change, albeit slight, in the glass composition due to volatilization from the glass melt Inhibiting this volatilization can reduce the change in the glass transition temperature Tg over time. The level of volatilization can be adjusted by adjusting the quantities of various components incorporated into the optical glass, including BO_(1.5) and F. The quantities of BO_(1.5) and F that are incorporated into the optical glass have the greatest effect on the level of volatilization.

TABLE 2-13 Form Basic A B C D Upper limit 1 5 5 5 5 5 2 3 3 3 3 3 3 2 2 2 2 2 4 1 1 1 1 1 5 0.8 0.8 0.8 0.8 0.8 6 0.5 0.5 0.5 0.5 0.5 7 0.3 0.3 0.3 0.3 0.3 8 0.1 0.1 0.1 0.1 0.1 Lower limit 1 0.00000 0.00000 0.00000 0.00000 0.00000 The change in specific gravity over time: Δ(DELTA)specific gravity(/hr)

The change in specific gravity over time is a measurement value obtained from various measured values of the specific gravity and a sampling time interval by sampling the glass melt during melting, measuring the specific gravity of the glass sample, allowing a prescribed interval of time to elapse following the sampling, sampling the glass melt again, measuring the specific gravity of the glass sample, and repeating this operation. The change in specific gravity over time can be reduced by reducing excess B—O—B bonds and reducing BO_(1.5)×F/O, which is the quantity of volatile components. From such perspectives, the upper and lower limits of the change in specific gravity over time, Δspecific gravity(/hr), are suitably set as indicated below. The change in specific gravity over time is caused by the change, albeit slight, in the glass composition due to volatilization from the glass melt Inhibiting this volatilization can reduce the change in specific gravity over time. The level of volatilization can be adjusted by adjusting the quantities of various components incorporated into the optical glass, including BO_(1.5) and F. The quantities of BO_(1.5) and F that are incorporated into the optical glass have the greatest effect on the level of volatilization.

TABLE 2-14 Form Basic A B C D Upper limit 1 0.01 0.01 0.01 0.01 0.01 2 0.008 0.008 0.008 0.008 0.008 3 0.005 0.005 0.005 0.005 0.005 4 0.003 0.003 0.003 0.003 0.003 5 0.002 0.002 0.002 0.002 0.002 6 0.0015 0.0015 0.0015 0.0015 0.0015 7 0.001 0.001 0.001 0.001 0.001 8 0.0005 0.0005 0.0005 0.0005 0.0005 Lower limit 1 0.00000 0.00000 0.00000 0.00000 0.00000 The change in the liquidus temperature over time: Δ(DELTA)LT(° C./hr)

The change in the liquidus temperature over time is a measurement value obtained from various measured values of the liquidus temperature and a sampling time interval by sampling the glass melt during melting, measuring the liquidus temperature of the glass sample, allowing a prescribed interval of time to elapse following the sampling, sampling the glass melt again, measuring the liquidus temperature of the glass sample, and repeating this operation. The change in liquidus temperature over time can be reduced by reducing excess B—O—B bonds and reducing BO_(1.5)×F/O, which is the quantity of volatile components. From such perspectives, the upper and lower limits of the change in liquidus temperature over time, ΔLT (° C./hr), are suitably set as indicated below. The change in liquidus temperature over time is caused by the change, albeit slight, in the glass composition due to volatilization from the glass melt Inhibiting this volatilization can reduce the change in the liquidus temperature over time. The level of volatilization can be adjusted by adjusting the quantities of various components incorporated into the optical glass, including BO_(1.5) and F. The quantities of BO_(1.5) and F that are incorporated into the optical glass have the greatest effect on the level of volatilization.

TABLE 2-15 Form Basic A B C D Upper 1 10 10 10 10 10 limit 2 8 8 8 8 8 3 6 6 6 6 6 4 4 4 4 4 4 5 3 3 3 3 3 6 2 2 2 2 2 7 1 1 1 1 1 Lower 0.00000 0.00000 0.00000 0.00000 0.00000 limit 1 The change in kinematic viscosity over time: Δ(DELTA)kinematic viscosity (m³/s/hr)

The change in the kinematic viscosity over time is a measurement value obtained from various measured values of the kinematic viscosity and a sampling time interval by sampling the glass melt during melting, measuring the kinematic viscosity of the glass sample, allowing a prescribed interval of time to elapse following the sampling, sampling the glass melt again, measuring the kinematic viscosity of the glass sample, and repeating this operation. The change in kinematic viscosity over time can be reduced by reducing excess B—O—B bonds and reducing BO_(1.5)×F/O, which is the quantity of volatile components. From such perspectives, the upper and lower limits of the change in kinematic viscosity over time, Akinematic viscosity (m³/s/hr), are suitably set as indicated below. The change in kinematic viscosity over time is caused by the change, albeit slight, in the glass composition due to volatilization from the glass melt Inhibiting this volatilization can reduce the change in the kinematic viscosity over time. The level of volatilization can be adjusted by adjusting the quantities of various components incorporated into the optical glass, including BO_(1.5) and F. The quantities of BO_(1.5) and F that are incorporated into the optical glass have the greatest effect on the level of volatilization.

TABLE 2-16 Form Basic A B C D Upper limit 1 1.0 1.0 1.0 1.0 1.0 2 0.8 0.8 0.8 0.8 0.8 3 0.6 0.6 0.6 0.6 0.6 4 0.4 0.4 0.4 0.4 0.4 5 0.2 0.2 0.2 0.2 0.2 6 0.1 0.1 0.1 0.1 0.1 7 0.05 0.05 0.05 0.05 0.05 8 0.02 0.02 0.02 0.02 0.02 9 0.01 0.01 0.01 0.01 0.01 Lower limit 1 0.00000 0.00000 0.00000 0.00000 0.00000 The refractive index in the present invention is represented by the refractive index nd in the d line. The fraction is represented by the Abbé number, νd.

The lower limit of the refractive index in the first and second basic forms of the present invention is, in order of increasing desirability, 1.70, 1.73, 1.75, 1.76, and 1.765. Achieving a high refractive index facilitates the achievement of high functionality and compactness in optical elements. The upper limit of the refractive index nd in the first and second basic forms of the present invention is, in order of increasing desirability, 1.90, 1.87, 1.83, 1.81, 1.79, and 1.78. Not employing an excessively high refractive index facilitates the maintaining of a low glass transition temperature and the maintaining of a glass with good thermal stability.

The lower limit of the Abbé number νd in the first and second basic forms of the present invention is, in order of increasing desirability, 36, 39, 43, 46, 48, 49, and 49.3. The use of a glass with low dispersion permits the realization of an optical glass that is better suited to the correction of chromatic aberration. The lower upper limit of the Abbé number νd in basic optical glasses of the first and second forms of the present invention is, in order of increasing desirability, 55, 54, 53, 52, 51, 50.5, 50, and 49.8. This is because an excessively high Abbé number tends to make it difficult to maintain the thermal stability of the glass and low glass transition temperature characteristics.

Form A is an optical glass having a refractive index nd of 1.75 to 1.81 and an Abbé number νd of 48 to 52.

Form B is an optical glass having a refractive index nd of 1.79 to 1.835 and an Abbé number νd of 42 to 48.

Form C is an optical glass having a refractive index nd of 1.675 to 1.76 and an Abbé number νd of 51 to 58.

Form D is an optical glass having a refractive index nd of 1.825 to 1.90 and an Abbé number νd of 35 to 43.

Desirable refractive indexes nd and Abbé numbers νd for the various forms are indicated below.

TABLE 2-17 Refractive index nd Form A B C D Upper limit 1 1.81 1.835 1.76 1.90 2 1.80 1.825 1.755 1.895 3 1.795 1.82 1.75 1.89 4 1.79 1.815 1.745 1.885 5 1.785 1.81 1.74 6 1.78 1.805 1.735 Lower limit 1 1.750 1.79 1.675 1.825 2 1.755 1.795 1.680 1.83 3 1.76 1.8 1.690 1.835 4 1.765 1.802 1.700 1.84 5 1.803 1.710 1.845 6 1.720 1.855 7 1.725 1.865 8 1.875

TABLE 2-18 Abbé number νd Form A B C D Upper limit 1 52.0 48 58.0 43.0 2 51.0 47.5 57.0 42.0 3 50.5 47 56.0 41.5 4 50.0 46.8 55.0 41.0 5 49.8 46.7 54.7 40.5 6 46.6 54.4 40 54.2 39 7 38 Lower limit 1 48 42 51 35 2 48.5 44 52 35.5 3 49 45 52 36 4 49.3 45.5 53 36.5 5 46 53 37 6 46.3 54

Both the above desirable refractive indexes and Abbé numbers can be realized in optical glasses that suitably combine any of the composition ranges specified in Tables 1-1 to 1-28 above and any of the characteristic ranges specified in Tables 2-1 to 2-18.

The optical glasses of the first basic form, second basic form, and Forms A, B, C, and D of the present invention can be obtained by procuring starting materials such as boric acid, oxides, and as needed, carbonates, nitrates, sulfates, and hydroxides; weighing out to achieve the desired characteristics, blending, and thoroughly stirring the various starting materials; charging the mixture to a melting vessel such as a platinum crucible; heating, melting, clarifying, and homogenizing the mixture; and molding the melt. To obtain a homogeneous glass, a melting temperature of, for example, 1,100 to 1,400° C. is desirable.

Alternatively, starting materials such as boric acid, oxides, and as needed, carbonates, nitrates, sulfates, and hydroxides can be rough melted to fabricate various cullets; the cullets can be blended to obtain desired characteristics, heated, melted, clarified, and homogenized; and the melt can be molded.

[The Preform for Precision Press-Molding]

The present invention includes a preform for precision press molding that is comprised of an optical glass of the first basic form, second basic form, or Form A, B, C, or D of the present invention.

A precision press-molding preform is a premolded glass material for use in the widely known precision press molding method.

The “precision press-molding preform” will sometimes be referred to simply as a “preform” hereinafter. The term “preform” refers to a premolded glass member that is heated for use in precision press molding. Here, the term “precision press-molding” is the method of forming the optically functional surfaces of an optical element by transfer from the molding surfaces of a pressing mold, also called mold optics molding, as is widely known. In optics, the term “optically functional surface” means a surface that refracts, reflects, diffracts, or permits the entering or exiting of, light that is being controlled. The surface of a lens or the like corresponds to such an optically functional surface.

During precision press molding, to permit good extension of the glass along the molding surface while preventing reaction and fusion of the glass to the pressing mold, the surface of the preform is desirably coated with a mold release film. Mold release films come in the form of noble metals (platinum, platinum alloys), oxides (oxides of Si, Al, Zr, La, Y, and the like), nitrides (nitrides of B, Si, Al, and the like), and carbon-containing films. A carbon-containing film comprised chiefly of carbon (in which the carbon content is greater than the content of other elements when the contents of the elements in the film are denoted as atomic percentages) is desirable as a carbon-containing film. Specific examples are carbon films and hydrocarbon films. A known method employing carbon as starting material, such as vacuum deposition, sputtering, and ion plating; or a known method employing a hydrocarbon or the like as the material gas, such as thermal decomposition, may be employed to form a carbon-containing film. Other films may be formed by deposition, sputtering, ion plating, the sol gel method, or the like.

The method of fabricating the preform of the present invention is not limited. However, it is desirably manufactured by the following methods utilizing the advantageous properties of the above-described glasses.

The first method of manufacturing a precision press-molding preform (referred to as preform manufacturing method I) is a method for manufacturing a precision press-molding preform by melting a glass starting material, causing the glass melt obtained to flow out, separating a glass melt gob, and molding the glass melt gob in a cooling step, and is characterized by the molding of a preform comprised of the above-described optical glass.

The second method of manufacturing a precision press-molding preform (referred to as preform manufacturing method II) is a method for manufacturing a precision press-molding preform by melting a glass starting material, molding the glass melt obtained to fabricate a molded glass member, and processing the molded member to fabricate a preform comprised of the optical glass of the present invention.

Both preform manufacturing methods I and II share the step of fabricating a homogenous glass melt from a glass starting material. For example, glass starting materials blended to yield desired characteristics are charged to a melting vessel made of platinum, and heated, melted, clarified, and homogenized to prepare a homogeneous glass melt. The glass melt is then caused to flow out of an outflow nozzle or an outflow pipe made of platinum or a platinum alloy the temperature of which has been adjusted. It is also possible to rough melt the glass starting materials to fabricate cullets; blend the cullets obtained; heat, melt, clarify, and homogenize to obtain a homogeneous glass melt; and cause the glass melt to flow out through the above-mentioned outflow nozzle or outflow pipe.

When forming small preforms or spherical preforms, the glass melt is made to drip out of the outflow nozzle in the form of a glass melt droplet of desired weight. This is then received in a preform-forming mold and molded into a preform. Alternatively, a glass melt droplet of desired weight is similarly made to drip out through an outflow nozzle into liquid nitrogen or the like to form a preform. When fabricating medium- or large-sized preforms, a glass melt flow is made to flow out of an outflow pipe, the front end of the glass melt flow is received by a preform-forming mold, a constriction is formed between the nozzle of the glass melt flow and the preform-forming mold, and the preform-forming mold is quickly lowered directly downward, allowing the surface tension of the glass melt to separate the glass melt flow at the site of the constriction. A glass melt gob of prescribed weight is then received in a receiving member and molded into a preform.

In order to fabricate a preform having a smooth surface that is free of scratches, grime, wrinkles, surface deformation, and the like, such as a free surface, it is possible to employ a method such as molding a glass melt gob into a preform while it is being floated by air pressure on a preform-forming mold or the like, or a method in which a glass melt droplet is placed in a medium such as liquid nitrogen that has been rendered liquid by cooling a gaseous material at the ordinary temperature and pressure, whereby a preform is molded.

When molding a glass melt gob into a preform while it is being floated, a gas (referred to as a “floating gas”) is blown onto the glass melt gob to apply upward air pressure. In this process, if the viscosity of the glass melt gob is excessively low, the floating gas penetrates into the glass, forming bubbles that remain in the preform. However, imparting a viscosity of 3 to 60 dPa·s to the glass melt gob floats the glass gob without the floating gas penetrating into the glass.

Examples of gases that can be employed in the course of blowing floating gas onto a preform are: air, N₂ gas, O₂ gas, Ar gas, He gas, steam and the like. The air pressure is not specifically limited other than that it be adequate to float the preform without it coming into contact with solid objects such as the surface of the forming mold.

The precision press-molded article (such as an optical element) that is fabricated from a preform will often have an axis of rotational symmetry such as is found in a lens. Thus, the shape of the preform desirably has an axis of rotational symmetry. Specific examples are spheres and items having a single axis of rotational symmetry. Examples of shapes having a single axis of rotational symmetry are items having smooth contour lines, free of edges and pits, in a cross-section containing the axis of rotational symmetry, such as an item having an ellipse the short axis of which conforms to the axis of rotational symmetry in the above cross-section. A shape in the form of an oblate sphere (a shape in which one axis running through the center of the sphere is established, and the dimensions are contracted in the direction of this axis) is another example.

In preform manufacturing method 1, the glass is molded within a temperature range permitting plastic deformation. Thus, a preform can be manufactured by press molding a glass gob. In that case, the shape of the preform can be set relatively freely, and can thus approximate the shape of the targeted optical element. For example, molding is possible where one of two opposing surfaces is made convex and the other concave, both surfaces are made concave, one surface is made flat and the other convex, one surface is made flat and the other concave, and both surfaces are made convex.

The surface of a preform thus fabricated can be coated with a carbon-containing film. However, the fluorine-containing optical glass of the present invention has a low glass transition temperature, permitting a low press molding temperature. Thus, it is not necessary to apply a carbon-containing film.

In preform manufacturing method 2, for example, after molding the glass melt by casting in a casting mold, strain of the molded member is removed by annealing, the molded member is cut or severed to split it into prescribed dimensions or shapes, multiple glass pieces are fabricated, and the glass pieces are polished to render their surfaces smooth, yielding preforms comprised of a prescribed weight of glass. The surface of a preform thus fabricated is also desirably coated with a carbon-containing film for use. Preform manufacturing method 2 is suited to the manufacturing of spherical preforms, platelike preforms, and the like that can be readily ground and polished.

In both of these manufacturing methods, the thermal stability of the optical glass employed is good. This permits the stable manufacturing of high-quality preforms with a tendency not to produce products that are defective because of glass devitrification, striae, or the like, and enhances the overall large-quantity production properties of the process of manufacturing optical elements.

Preforms that are preferred because they enhance the large-quantity production properties of optical elements by precision press molding will be described next.

From the aspect of a glass material, the optical glass of the present invention provides excellent precision press-molding properties. Reducing the amount of deformation of the glass in precision press molding permits lowering the temperature of the glass and the pressing mold during precision press molding, reducing the time required for press molding, and reducing the pressure applied during pressing. As a result, reactivity between the glass and the surface of the pressing mold decreases, the above problems occurring during precision press molding decrease, and large-quantity production properties improve.

The preforms that are desirably employed when precision press molding lenses have mutually opposing pressed surfaces (surfaces that are pressed by opposing molding surfaces in the mold during precision press molding). Preforms having an axis of rotational symmetry that runs through the center of the two pressed surfaces are preferred. Among such preforms, those that are suited to the precision press molding of meniscus lenses are those in which one pressed surface is convex and the other is concave, flat, or convex having a curvature that is less than the first convex surface.

In preforms that are suited to the precision press molding of biconcave lenses, one pressed surface is convex, concave, or flat, and the other surface is convex, concave, or flat.

In preforms that are suited to the precision press molding of biconvex lenses, one pressed surface is convex and the other surface is convex or flat.

In both cases, a preform with a shape approximating the shape of the article being precision press molded is desirable.

When molding a glass melt gob into a preform with a preform-forming mold, the lower surface of the glass on the pressing mold is approximately determined by the shape of the molding surface of the pressing mold. Additionally, the shape of the upper surface of the glass is determined by the surface tension of the glass melt and the weight of the glass itself. To reduce the amount of deformation of the glass during precision press molding, it is necessary to also control the shape of the upper surface of the glass during molding in the preform-forming mold. The shape of the upper surface of the glass determined by the surface tension of the glass melt and the weight of the glass itself is a free surface that is convex in shape. However, pressure can be applied to the upper surface of the glass to render it flat, concave, or convex with less curvature than the free surface. Specifically, the upper surface of the glass is molded to the desired shape by pressing with a pressing mold having a molding surface of desired shape, or by applying air pressure to the upper surface of the glass. In the course of pressing the upper surface of the glass with a pressing mold, multiple gas blow-holes can be provided in the molding surface of the forming mold, gas can be blown through the gas blow-holes to form a gas cushion between the molding surface and the upper surface of the glass, and the upper surface of the glass can be pressed through this gas cushion. Alternatively, when it is desirable to mold the upper surface of the glass into a surface of greater curvature than the free surface, negative pressure can be generated in the vicinity of the upper surface of the glass to mold the upper surface in a more mounded fashion.

A preform with a polished surface is also desirable to obtain a preform with a shape more closely approximating the shape of the article being precision press molded. For example, a preform in which one pressed surface is polished to be either flat or partially spherical, and the other pressed surface is polished so as to be partially spherical or flat, is desirable. The partially spherical portion of the surface can be either convex or concave. Whether to make it convex or concave is desirably determined based on the shape of the article being precision press molded, as set forth above.

The various preforms described above are desirable for molding lenses with diameters of 10 mm or greater and preferred for molding lenses with diameters of 20 mm or greater. They are also desirable for molding lenses with a center thickness exceeding 2 mm.

[The Optical Element]

The optical element of the present invention will be described next.

The optical element of the present invention is comprised of the optical glass of the above-described first basic form, second basic form, or Form A, B, C, or D.

The type of optical element itself is not specifically limited. Examples of typical elements are lenses such as aspherical lenses, spherical lenses, plano-concave lenses, plano-convex lenses, biconcave lenses, biconvex lenses, convex meniscus lenses, and concave meniscus lenses; microlenses; lens arrays; lenses with diffraction gratings; prisms; and prisms with lens functions. Examples of desirable optical elements are lenses such as convex meniscus lenses, concave meniscus lenses, biconvex lenses, biconcave lenses, plano-convex lenses, and plano-concave lenses; prisms; and diffraction gratings. Each of these lenses may be asymmetrical or spherical. As required, an antireflective coating, partially reflective coating with wavelength selecting properties, or the like may be provided on the surface.

The optical element of the present invention is comprised of glass with a high refractive index and low dispersion characteristics. It can thus be combined with optical elements comprised of other glasses to achieve good correction of chromatic aberration. Since the optical element of the present invention is comprised of a glass of high refractive index, it can be employed in image pickup optical systems, projection optical systems, and the like to render the optical system more compact.

The method of manufacturing the optical element of the present invention will be described next.

The optical element of the present invention can be manufactured by precision press molding the above-described preform of the present invention with a pressing mold.

A desirable form of the method of manufacturing the optical element of the present invention is the method of large-quantity production of optical elements by repeating the step of precision press molding the above-described preform of the present invention in a single pressing mold.

To prevent oxidation of the mold release film provided on the molding surfaces of the pressing mold or the molded surfaces, the step of heating the pressing mold and preform and the precision press molding step are desirably conducted in a non-oxidizing gas atmosphere such as nitrogen gas or a mixed gas of nitrogen gas and hydrogen gas. In a non-oxidizing gas atmosphere, the carbon-containing film coated on the preform surface will remain on the surface of the precision press molded article without oxidizing. This film is eventually to be removed. To relatively easily and completely remove the carbon-containing film, it suffices to heat the precision press molded article in an oxidizing atmosphere, such as air. The oxidation and removal of the carbon-containing film is to be conducted at a temperature at which the precision press molded article will not be deformed by heating. Specifically, this is desirably conducted within a temperature range below the glass transition temperature.

A pressing mold the molding surfaces of which have been processed with high precision into desired shapes in advance is employed in precision press molding. A mold release film can be formed on the molding surfaces to prevent fusion of the glass during pressing. Examples of the mold release film are carbon-containing films, nitride films, and noble metal films. Hydrogenated carbon films, carbon films, and the like are desirable as carbon-containing films.

More specifically, in addition to SiC molds, superhard molds such as WC, molds made of cermets, and the like can be employed as the pressing mold in precision press molding because the glass transition temperature of the optical glass of the present invention is low. As needed, the molding surfaces made of an SiC mold can be coated with a carbon film. As needed, the molding surfaces of a mold made of a superhard material such as WC can be coated with a noble metal film or noble metal alloy film.

The method of manufacturing the optical element comprises the following two forms.

In the first form of the method of manufacturing an optical element by precision press molding (referred to as optical element manufacturing method I), a preform is introduced into the pressing mold and the preform and the pressing mold are heated together. In the second form of the method of manufacturing an optical element by precision press molding (referred to as optical element manufacturing method II), a heated preform is introduced into a preheated pressing mold.

In optical element manufacturing method I, the preform is positioned between a pair of opposing upper and lower molds the molding surfaces of which have been processed into precise shapes, after which both the pressing molds and the preform are heated to a temperature corresponding to a glass viscosity of 10⁵ to 10⁹ dPa·s to soften the preform. The preform is then press molded to precisely transfer the molding surfaces of the pressing molds to the glass. Optical element manufacturing method I is recommended when the focus is on improving molding precision such as surface precision and eccentric precision.

In optical element manufacturing method II, a preform that has been preheated to a temperature corresponding to a glass viscosity of 10⁴ to 10⁸ dPa·s is positioned between a pair of opposing upper and lower molds the molding surfaces of which have been processed into precise shapes and press molded to precisely transfer the molding surfaces of the pressing molds to the glass. Optical element manufacturing method II is recommended when the focus is on increasing productivity.

The pressure and duration of pressing can be suitably determined taking into account the viscosity and the like of the glass. For example, a pressure of about 5 to 15 MPa and a pressing time of 10 to 300 seconds can be employed. Pressing conditions such as the pressing time and pressure can be suitably set to within known ranges in accordance with the shape and dimensions of the molded product.

Subsequently, the pressing mold and precision press molded product are cooled, desirably to a temperature below the strain point, the molds are separated, and the precision press-molded product is removed. To achieve optical characteristics conforming precisely to desired values, it suffices to suitably adjust the annealing conditions of the molded product during cooling, such as the annealing rate.

The optical element of the present invention can be fabricated without a press molding step. For example, a homogeneous glass melt can be cast in a casting mold to form a glass block and annealed to remove strain. Optical characteristics can be adjusted by regulating the annealing conditions to achieve a desired value for the refractive index of the glass. Subsequently, the glass block can be cut or severed to obtain glass pieces, which can then be ground and polished to obtain finished glass elements.

The lens unit of the present invention is assembled into a body of equipment in the form of an optical system comprising optical elements of the present invention, preferably asymmetrical or symmetrical lenses. It can be an interchangeable lens in a single-lens reflex camera, a lens unit built into a compact camera, or the like.

The image pickup device of the present invention is equipped with the above-described optical element of the present invention. Since the optical element of the present invention is comprised of the optical glass having a high refractive index and low dispersion characteristics, a lens with a high dispersion characteristic can be combined with a lens in the form of the optical element of the present invention to provide an image pickup device having a compact image pickup optical system capable of good correction of chromatic aberration. Image sensors such as CCDs and CMOSs can be employed as the image pickup elements constituting the image pickup device.

That is, the image pickup device of the present invention is equipped with at least the above lens unit and image sensors such as CCDs or CMOSs. As needed, a mechanism having a focusing (focus adjusting) function can be provided.

EXAMPLES

The present invention will be described in greater detail below through examples.

Examples

Examples 1 to 33 in Table 3 are mainly examples of Form A. Examples 1 to 44 in Table 4 are mainly examples of Form B. Examples 1 to 28 in Table 5 are mainly examples of Form C. And Examples 1 to 9 in Table 6 are mainly examples of Form D.

Glass starting materials were dispensed in the form of boric acid or boric anhydride, oxides, fluorides, and, as needed, carbonates, nitrates, and the like in a manner calculated to yield optical glasses having the compositions indicated in Tables 3 to 6; charged to a platinum crucible; heated in air, desirably with a platinum cover in place; melted, clarified and homogenized for 1.5 to 3 hours at 1,150 to 1,250° C.; charged to a preheated casting mold; molded; and gradually cooled to obtain glass blocks. No crystal precipitation or striae were observed in the glass. The glass thus obtained was used to measure the various characteristics of glass by the methods outlined in a separate table. The results obtained are given in Tables 3 to 6. The reduction in weight of Example 14 of Table 5, which was of Form C, was 0.61 weight percent. Similarly, the reduction in weight of Example 18 of Table 5 was 1.1 weight percent.

Examples 32 and 33 (Form A) of Table 3 are the glass characteristics obtained by melting for two and a half hours and the various characteristics of other examples given in Tables 3 to 6 are glass characteristics obtained by melting for two hours.

TABLE 3 Example Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 BO_(1.5) 49.73 49.73 47.80 46.62 45.46 43.48 SiO₂ 5.52 5.52 7.55 7.36 6.99 8.45 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 11.05 11.05 10.06 9.82 9.32 8.45 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 4.42 4.42 5.03 4.91 6.41 7.25 LaO_(1.5) 13.27 13.27 12.58 13.50 13.41 14.49 GdO_(1.5) 12.15 12.15 12.58 13.50 14.57 14.49 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 3.31 3.31 3.77 3.68 2.91 2.42 TaO_(2.5) 0.55 0.55 0.63 0.61 0.93 0.97 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 8.84 17.68 5.03 9.82 13.99 14.49 O (Note 3) 137.285 132.865 141.2 138.945 136.36 137.085 F/O(Molar ratio) 0.064 0.133 0.036 0.071 0.103 0.106 Glass transition temperature Tg (° C.) 562 548 578 568 557 558 Liquidus temperature LT (° C.) 1080 1070 1090 1090 1070 1060 Specific gravity 4.66 4.66 4.67 4.61 4.88 4.91 Viscosity at liquidus temperature 2.5 3.0 2.4 2.4 2.9 3.3 (dPa · s) Refractive index nd 1.76662 1.75392 1.77209 1.77448 1.77356 1.77270 Abbé number νd 49.91 50.84 49.24 49.30 49.40 49.44 nd − (2.25 − 0.01 × νd) 0.016 0.012 0.014 0.017 0.018 0.017 λ80 (nm) 367 364 366 366 362 365 λ70 (nm) 345 343 344 344 340 344 λ5 (nm) 276 275 277 276 278 280 BO_(1.5) × F/O 3.20 6.62 1.70 3.29 4.66 4.60 Excess B—O—B Bonding:D 11.9 16.3 2.2 0.9 0.0 −6.6 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.111 0.111 0.158 0.158 0.154 0.194 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 55.25 55.25 55.35 53.98 52.45 51.93 ΣRE 25.41 25.41 25.16 27.00 27.97 28.98 LaO_(1.5)/ΣRE 0.522 0.522 0.500 0.500 0.479 0.500 ΣRE/NWF 0.460 0.460 0.455 0.500 0.533 0.558 2LiO_(0.5) + ZnO + (F/2) 30.94 35.36 27.67 29.46 32.04 31.40 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.833 0.833 0.800 0.800 0.744 0.700 ZnO + 3ZrO₂ + 5TaO_(2.5) 17.10 17.10 19.49 19.00 19.79 19.36 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 0.41 BO_(1.5) × F/LaO_(1.5) 16.56 33.13 9.56 16.96 23.71 21.74 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.86 0.86 0.86 0.86 0.76 0.71 ZrO₂ + TaO_(2.5) 3.86 3.86 4.40 4.29 3.84 3.39 Example Example 7 Example 8 Example 9 Example 10 Example 11 Example 12 BO_(1.5) 42.97 43.22 39.74 41.50 41.42 52.94 SiO₂ 8.35 8.40 11.54 10.06 7.84 3.53 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 5.97 7.20 11.54 10.06 4.48 4.71 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 11.93 7.20 6.41 8.81 13.44 6.47 LaO_(1.5) 14.32 15.31 13.46 13.21 15.68 15.29 GdO_(1.5) 14.32 15.31 13.46 13.21 15.68 12.94 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 1.19 2.40 3.21 2.52 0.56 3.53 TaO_(2.5) 0.95 0.96 0.64 0.63 0.90 0.59 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 14.32 19.21 7.69 7.55 22.4 14.12 O (Note 3) 136.625 135.955 139.425 138.68 132.7 139.115 F/O(Molar ratio) 0.105 0.141 0.055 0.054 0.169 0.101 Glass transition temperature Tg (° C.) 562 564 563 559 555 579 Liquidus temperature LT (° C.) 1070 1060 1120 1070 1050 1070 Specific gravity 4.95 4.99 4.75 4.77 5.10 4.87 Viscosity at liquidus temperature 3.0 3.4 1.9 2.9 3.7 3.0 (dPa · s) Refractive index nd 1.77221 1.77473 1.77021 1.77113 1.77039 1.77960 Abbé number νd 49.31 49.60 48.93 49.09 49.70 49.61 nd − (2.25 − 0.01 × νd) 0.015 0.021 0.010 0.012 0.017 0.026 λ80 (nm) 366 366 363 362 364 366 λ70 (nm) 343 344 342 342 342 344 λ5 (nm) 279 277 277 278 278 275 BO_(1.5) × F/O 4.50 6.11 2.19 2.26 6.99 5.37 Excess B—O—B Bonding:D −7.7 −6.3 −20.2 −14.2 −8.4 19.7 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.194 0.194 0.290 0.242 0.189 0.067 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 51.32 51.62 51.28 51.56 49.26 56.47 ΣRE 28.64 30.62 26.92 26.42 31.36 28.23 LaO_(1.5)/ΣRE 0.500 0.500 0.500 0.500 0.500 0.542 ΣRE/NWF 0.558 0.593 0.525 0.512 0.637 0.500 2LiO_(0.5) + ZnO + (F/2) 31.03 31.20 33.34 32.70 33.60 22.95 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.500 0.667 0.783 0.695 0.400 0.593 ZnO + 3ZrO₂ + 5TaO_(2.5) 20.25 19.20 19.24 19.52 19.62 20.01 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 1.21 BO_(1.5) × F/LaO_(1.5) 21.49 27.11 11.35 11.86 29.59 24.44 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.56 0.71 0.83 0.80 0.38 0.86 ZrO₂ + TaO_(2.5) 2.14 3.36 3.85 3.15 1.46 4.12 Example Example 13 Example 14 Example 15 Example 16 Example 17 Example 18 BO_(1.5) 43.47 43.48 42.46 45.46 45.46 42.71 SiO₂ 8.45 3.62 8.25 6.99 6.99 8.30 GeO₂ 0.00 4.83 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 8.45 8.45 2.36 8.16 8.16 2.37 NaO_(0.5) 0.00 0.00 0.00 1.17 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 1.17 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 1.19 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 7.25 7.25 16.51 6.41 6.41 13.05 LaO_(1.5) 12.08 14.49 14.74 13.40 13.40 15.42 GdO_(1.5) 12.08 14.49 14.74 14.57 14.57 15.42 YO_(1.5) 4.83 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.42 2.42 0.00 2.91 2.91 0.59 TaO_(2.5) 0.97 0.97 0.94 0.93 0.93 0.95 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 14.49 14.49 16.51 13.99 13.99 18.98 O (Note 3) 137.085 137.085 136.195 136.35 136.35 136.415 F/O(Molar ratio) 0.106 0.106 0.121 0.103 0.103 0.139 Glass transition temperature Tg (° C.) 560 558 566 554 553 569 Liquidus temperature LT (° C.) 1110 1060 1060 1060 1060 1060 Specific gravity 4.76 5.00 5.04 4.84 4.86 5.06 Viscosity at liquidus temperature 2.0 3.1 3.4 3.2 3.2 3.4 (dPa · s) Refractive index nd 1.76758 1.78220 1.77393 1.76819 1.77085 1.77450 Abbé number νd 49.61 48.02 49.07 49.42 49.44 49.46 nd − (2.25 − 0.01 × νd) 0.014 0.012 0.015 0.012 0.015 0.019 λ80 (nm) 364 364 368 367 367 367 λ70 (nm) 343 341 346 345 346 345 λ5 (nm) 277 276 276 277 278 276 BO_(1.5) × F/O 4.59 4.60 5.15 4.66 4.66 5.94 Excess B—O—B Bonding:D −6.7 −6.6 −8.8 0.0 0.0 −8.3 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.194 0.083 0.194 0.154 0.154 0.194 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 51.92 51.93 50.71 52.45 52.45 51.01 ΣRE 28.99 28.98 29.48 27.97 27.97 30.84 LaO_(1.5)/ΣRE 0.417 0.500 0.500 0.479 0.479 0.500 ΣRE/NWF 0.558 0.558 0.581 0.533 0.533 0.605 2LiO_(0.5) + ZnO + (F/2) 31.40 31.40 29.48 29.72 29.72 27.28 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.700 0.700 0.222 0.718 0.718 0.266 ZnO + 3ZrO₂ + 5TaO_(2.5) 19.36 19.36 21.21 19.79 19.79 19.57 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 0.18 0.52 BO_(1.5) × F/LaO_(1.5) 26.07 21.74 23.78 23.73 23.73 26.29 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.71 0.71 0.00 0.76 0.76 0.38 ZrO₂ + TaO_(2.5) 3.39 3.39 0.94 3.84 3.84 1.54 Example Example 19 Example 20 Example 21 Example 22 Example 23 Example 24 BO_(1.5) 42.71 44.27 42.34 44.56 43.48 40.76 SiO₂ 8.30 8.61 9.96 8.66 8.45 7.93 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 2.37 9.23 9.34 8.66 8.45 2.27 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 1.19 0.00 0.00 0.00 0.00 0.00 ZnO 13.05 6.15 6.23 7.43 7.25 13.59 LaO_(1.5) 15.42 16.61 14.32 13.61 15.94 19.25 GdO_(1.5) 15.42 11.69 14.32 10.64 10.14 14.72 YO_(1.5) 0.00 0.00 0.00 2.97 2.90 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.59 2.46 2.49 2.48 2.42 0.57 TaO_(2.5) 0.95 0.98 1.00 0.99 0.97 0.91 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 18.98 12.3 12.45 14.85 14.49 27.18 O (Note 3) 136.415 138.06 138.545 136.76 137.085 132.505 F/O(Molar ratio) 0.139 0.089 0.09 0.109 0.106 0.205 Glass transition temperature Tg (° C.) 569 561 563 559 562 563 Liquidus temperature LT (° C.) 1060 1120 1080 1050 1070 1070 Specific gravity 5.08 4.79 4.84 4.69 4.76 5.18 Viscosity at liquidus temperature 3.4 1.9 2.7 3.6 2.9 3.0 (dPa · s) Refractive index nd 1.77379 1.77235 1.77142 1.76321 1.77105 1.77567 Abbé number νd 49.45 49.48 48.48 49.93 49.57 49.66 nd − (2.25 − 0.01 × νd) 0.018 0.017 0.006 0.012 0.017 0.022 λ80 (nm) 368 362 363 364 367 367 λ70 (nm) 347 342 342 343 344 346 λ5 (nm) 277 277 278 278 279 275 BO_(1.5) × F/O 5.94 3.94 3.80 4.84 4.60 8.36 Excess B—O—B Bonding:D −8.3 −5.2 −11.5 −3.1 −6.6 −10.2 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.194 0.194 0.235 0.194 0.194 0.195 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 51.01 52.88 52.30 53.22 51.93 48.69 ΣRE 30.84 28.30 28.64 27.22 28.98 33.97 LaO_(1.5)/ΣRE 0.500 0.587 0.500 0.500 0.550 0.567 ΣRE/NWF 0.605 0.535 0.548 0.512 0.558 0.698 2LiO_(0.5) + ZnO + (F/2) 27.28 30.76 31.14 32.18 31.40 31.72 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.266 0.750 0.750 0.700 0.700 0.250 ZnO + 3ZrO₂ + 5TaO_(2.5) 19.57 18.43 18.70 19.82 19.36 19.85 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 0.69 BO_(1.5) × F/LaO_(1.5) 26.29 16.39 18.41 24.31 19.76 28.78 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.38 0.72 0.71 0.71 0.71 0.39 ZrO₂ + TaO_(2.5) 1.54 3.44 3.49 3.47 3.39 1.48 Example Example 25 Example 26 Example 27 Example 28 Example 29 Example 30 BO_(1.5) 40.76 42.34 42.34 41.57 41.57 41.51 SiO₂ 7.93 9.96 9.96 9.78 9.78 10.06 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 2.27 9.34 9.34 8.56 8.56 10.06 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 13.59 6.23 6.23 7.33 7.33 8.81 LaO_(1.5) 14.72 9.34 19.30 13.21 14.67 11.32 GdO_(1.5) 19.25 19.30 9.34 13.20 11.74 11.32 YO_(1.5) 0.00 0.00 0.00 2.93 2.93 3.77 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.57 2.49 2.49 2.44 2.44 2.52 TaO_(2.5) 0.91 1.00 1.00 0.98 0.98 0.63 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 27.18 12.45 12.45 14.67 14.67 7.55 O (Note 3) 132.505 138.545 138.545 137.53 137.53 138.68 F/O(Molar ratio) 0.205 0.09 0.09 0.107 0.107 0.054 Glass transition temperature Tg (° C.) 565 567 557 560 559 553 Liquidus temperature LT (° C.) 1090 1100 1100 1070 1070 1080 Specific gravity 5.26 4.94 4.76 4.83 4.80 4.65 Viscosity at liquidus temperature 2.5 2.2 2.2 3.0 3.0 2.7 (dPa · s) Refractive index nd 1.77441 1.77058 1.77366 1.76989 1.77068 1.76737 Abbé number νd 49.80 49.59 49.34 49.41 49.36 49.36 nd − (2.25 − 0.01 × νd) 0.022 0.016 0.017 0.014 0.014 0.011 λ80 (nm) 368 366 364 364 364 362 λ70 (nm) 347 346 342 344 343 341 λ5 (nm) 277 278 274 276 276 278 BO_(1.5) × F/O 8.36 3.80 3.80 4.43 4.43 2.26 Excess B—O—B Bonding:D −10.2 −11.5 −11.5 −12.8 −12.8 −14.2 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.195 0.235 0.235 0.235 0.235 0.242 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 48.69 52.30 52.30 51.35 51.35 51.57 ΣRE 33.97 28.64 28.64 29.33 29.34 26.41 LaO_(1.5)/ΣRE 0.433 0.326 0.674 0.450 0.500 0.429 ΣRE/NWF 0.698 0.548 0.548 0.571 0.571 0.512 2LiO_(0.5) + ZnO + (F/2) 31.72 31.14 31.14 31.78 31.78 32.70 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.250 0.750 0.750 0.700 0.700 0.695 ZnO + 3ZrO₂ + 5TaO_(2.5) 19.85 18.70 18.70 19.55 19.55 19.52 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 37.63 28.22 13.66 23.08 20.79 13.84 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.39 0.71 0.71 0.71 0.71 0.80 ZrO₂ + TaO_(2.5) 1.48 3.49 3.49 3.42 3.42 3.15 Example Example 31 Example 32 Example 33 BO_(1.5) 41.72 45.51 46.62 SiO₂ 8.11 7.19 7.36 GeO₂ 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 LiO_(0.5) 4.63 9.58 9.82 NaO_(0.5) 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 MgO 0.00 0.00 0.00 CaO 0.00 0.00 0.00 SrO 0.00 0.00 0.00 BaO 0.00 0.00 0.00 ZnO 13.90 4.79 4.91 LaO_(1.5) 13.90 14.37 13.49 GdO_(1.5) 12.75 14.37 13.50 YO_(1.5) 3.48 0.00 0.00 TiO₂ 0.00 0.00 0.00 ZrO₂ 0.58 2.99 3.07 TaO_(2.5) 0.93 1.20 1.23 NbO_(2.5) 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 F (Note 2) 23.17 14.37 9.82 O (Note 3) 132.11 137.13 139.26 F/O(Molar ratio) 0.175 0.105 0.071 Glass transition temperature Tg (° C.) 548 562 566 Liquidus temperature LT (° C.) 1140 1090 1100 Specific gravity 4.93 4.89 4.89 Viscosity at liquidus temperature 1.6 2.5 2.4 (dPa · s) Refractive index nd 1.76871 1.77470 1.77700 Abbé number νd 49.65 49.34 49.21 nd − (2.25 − 0.01 × νd) 0.015 0.018 0.019 λ80 (nm) 364 364 367 λ70 (nm) 343 342 346 λ5 (nm) 278 277 279 BO_(1.5) × F/O 7.32 4.77 3.29 Excess B—O—B Bonding:D −6.9 −0.6 0.6 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.194 0.158 0.158 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 49.83 52.70 53.98 ΣRE 30.13 28.74 27.00 LaO_(1.5)/ΣRE 0.461 0.500 0.500 ΣRE/NWF 0.605 0.545 0.500 2LiO_(0.5) + ZnO + (F/2) 34.75 31.14 29.46 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.400 0.800 0.800 ZnO + 3ZrO₂ + 5TaO_(2.5) 20.29 19.76 20.27 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 34.77 22.76 16.97 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.38 0.71 0.71 ZrO₂ + TaO_(2.5) 1.51 4.19 4.30 (Note 1) The contents of BO_(1.5) to SbO_(1.5) are given as cationic percentages. (Note 2) F is the relative quantity (in moles) of fluorine ions for the total quantity of cations, denoted as 100. (Note 3) O is the relative quantity (in moles) of oxygen ions for the total quantity of cations, denoted as 100. (Note 4) Volatilization peak Tv is the temperature (° C.) of the endothermic peak accompanying change in the discontinuous weight reduction rate of the glass when heated to 1,200° C. at 10° C./min. (Note 5) The weight reduction was the amount of weight reduction (wt %) when 100 mg of glass was heated from 400° C. to 1,200° C. at 10° C./min.

TABLE 4 Example Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 BO_(1.5) 47.60 42.10 44.56 40.07 36.46 35.11 32.92 SiO₂ 4.80 6.32 3.96 7.38 10.55 10.16 10.65 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 4.80 5.26 7.92 7.38 8.63 10.16 9.69 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 11.40 12.63 11.88 8.43 7.67 9.24 9.69 LaO_(1.5) 15.20 15.79 14.85 18.98 18.99 18.29 19.18 GdO_(1.5) 9.50 10.53 9.90 11.28 11.41 10.99 11.53 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.90 3.16 2.97 2.90 2.64 2.54 2.66 TaO_(2.5) 3.80 4.21 3.96 3.58 3.65 3.51 3.68 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 8.00 8.42 7.92 16.87 15.35 14.78 15.50 O (Note 3) 143.15 143.17 139.61 138.69 140.11 137.69 138.05 F/O(Molar ratio) 0.056 0.059 0.057 0.122 0.110 0.107 0.112 Glass transition temperature 574 570 549 567 562 550 551 Tg (° C.) Liquidus temperature LT (° C.) 1070 1090 1090 1090 1130 1130 1120 Specific gravity 4.96 5.11 5.02 5.14 5.10 5.08 5.17 Viscosity at liquidus temperature 5.8 4.6 4.0 3.3 2.4 2.3 2.6 (dPa · s) Refractive index nd 1.80178 1.81460 1.80670 1.80453 1.80188 1.80034 1.80547 Abbé number νd 45.87 44.88 45.24 46.34 46.35 46.21 45.92 nd − (2.25 − 0.01 × νd) 0.010 0.013 0.009 0.018 0.015 0.012 0.015 λ80 (nm) 380 382 379 376 376 373 378 λ70 (nm) 356 356 354 352 352 349 352 λ5 (nm) 285 284 283 282 281 280 281 BO_(1.5) × F/O 2.67 2.48 2.54 4.89 4.01 3.76 3.69 Excess B—O—B Bonding:D −0.3 −16.9 −5.9 −18.5 −30.7 −32.4 −39.3 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.101 0.150 0.089 0.184 0.289 0.289 0.324 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + 52.40 48.42 48.52 47.45 47.01 45.27 43.57 GeO₂) ΣRE 24.70 26.32 24.75 30.26 30.40 29.28 30.71 LaO_(1.5)/ΣRE 0.615 0.600 0.600 0.627 0.625 0.625 0.625 ΣRE/NWF 0.471 0.544 0.510 0.638 0.647 0.647 0.705 2LiO_(0.5) + ZnO + (F/2) 25.00 27.36 31.68 31.62 32.60 36.95 36.82 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.457 0.454 0.571 0.636 0.692 0.687 0.667 ZnO + 3ZrO₂ + 5TaO_(2.5) 39.10 43.16 40.59 35.03 33.84 34.41 36.07 Volatilization peak Tv (° C.) >1200 >1200 1104 1164 1084 1150 1128 (Note 4) Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 12.53 11.22 11.88 17.81 14.74 14.19 13.30 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.43 0.43 0.43 0.45 0.42 0.42 0.42 ZrO₂ + TaO_(2.5) 6.70 7.37 6.93 6.48 6.29 6.05 6.34 Example Example 8 Example 9 Example 10 Example 11 Example 12 Example 13 BO_(1.5) 34.34 35.38 49.05 49.59 47.39 46.86 SiO₂ 11.14 10.24 1.09 1.10 1.10 1.08 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 8.35 8.38 3.26 3.29 3.30 3.30 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 9.28 9.31 13.08 13.23 13.22 13.08 LaO_(1.5) 18.84 18.90 15.69 15.21 15.87 15.69 GdO_(1.5) 11.74 11.78 10.47 10.14 10.58 10.46 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.97 2.56 3.00 3.03 3.03 2.99 TaO_(2.5) 3.34 3.45 4.36 4.41 4.41 4.36 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 1.10 2.18 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 14.85 14.90 17.44 17.64 17.63 17.43 O (Note 3) 139.98 139.37 137.89 137.75 139.40 141.11 F/O(Molar ratio) 0.106 0.107 0.126 0.128 0.126 0.124 Glass transition temperature Tg (° C.) 561 560 569 568 565 565 Liquidus temperature LT (° C.) 1110 1090 1090 1090 1080 1100 Specific gravity 5.13 5.14 5.18 5.15 5.27 5.31 Viscosity at liquidus temperature 3.1 3.9 3.0 2.9 3.3 2.7 (dPa · s) Refractive index nd 1.80431 1.80417 1.81301 1.81036 1.82374 1.82991 Abbé number νd 46.25 46.27 45.32 45.25 43.52 42.30 nd − (2.25 − 0.01 × νd) 0.017 0.017 0.016 0.013 0.009 0.003 λ80 (nm) 375 375 380 376 389 396 λ70 (nm) 350 350 353 349 364 370 λ5 (nm) 280 280 280 279 330 336 BO_(1.5) × F/O 3.64 3.79 6.18 6.35 5.97 5.81 Excess B—O—B Bonding:D −37.0 −33.2 9.3 11.0 2.8 −0.5 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.324 0.289 0.022 0.022 0.023 0.023 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 45.48 45.62 50.14 50.69 48.49 47.94 ΣRE 30.58 30.68 26.16 25.35 26.45 26.15 LaO_(1.5)/ΣRE 0.616 0.616 0.600 0.600 0.600 0.600 ΣRE/NWF 0.672 0.673 0.522 0.500 0.545 0.545 2LiO_(0.5) + ZnO + (F/2) 33.40 33.52 28.32 28.63 28.63 28.40 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.643 0.643 0.333 0.332 0.333 0.335 ZnO + 3ZrO₂ + 5TaO_(2.5) 34.89 34.24 43.88 44.37 44.36 43.85 Volatilization peak Tv (° C.) (Note 4) 1036 1171 1115 1105 1114 1094 Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 13.53 13.95 27.26 28.76 26.32 26.03 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.47 0.43 0.41 0.41 0.41 0.41 ZrO₂ + TaO_(2.5) 6.31 6.01 7.36 7.44 7.44 7.35 Example Example 14 Example 15 Example 16 Example 17 Example 18 Example 19 BO_(1.5) 47.37 46.84 48.43 45.57 48.45 47.31 SiO₂ 1.11 1.09 0.00 2.28 0.00 2.28 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 3.30 3.30 3.38 3.43 3.37 3.41 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 13.23 13.08 13.53 13.67 13.86 14.35 LaO_(1.5) 15.87 15.69 16.23 16.41 16.23 16.41 GdO_(1.5) 10.58 10.46 10.82 10.94 10.82 10.94 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 1.10 2.18 0.00 0.00 0.00 0.00 ZrO₂ 3.03 3.00 3.10 3.14 2.76 2.45 TaO_(2.5) 4.41 4.36 4.51 4.56 4.51 2.85 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 17.64 17.44 18.04 18.23 18.03 18.23 O (Note 3) 138.30 138.94 136.90 137.89 136.58 135.52 F/O(Molar ratio) 0.128 0.126 0.132 0.132 0.132 0.135 Glass transition temperature Tg (° C.) 565 565 562 565 557 549 Liquidus temperature LT (° C.) 1100 1100 1100 1100 1100 1100 Specific gravity 5.21 5.20 5.25 5.25 5.25 5.12 Viscosity at liquidus temperature 2.6 2.7 2.5 2.7 2.5 2.7 (dPa · s) Refractive index nd 1.82431 1.83253 1.82100 1.81848 1.81804 1.79861 Abbé number νd 43.48 42.05 44.77 44.82 44.90 46.78 nd − (2.25 − 0.01 × νd) 0.009 0.003 0.019 0.017 0.017 0.016 λ80 (nm) 394 407 380 377 377 372 λ70 (nm) 363 370 351 349 349 346 λ5 (nm) 329 335 279 279 278 278 BO_(1.5) × F/O 6.06 5.90 6.39 6.02 6.40 6.39 Excess B—O—B Bonding:D 3.8 1.6 8.4 −1.2 8.8 6.4 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.023 0.023 0.000 0.050 0.000 0.048 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 48.48 47.93 48.43 47.85 48.45 49.59 ΣRE 26.45 26.15 27.05 27.35 27.05 27.35 LaO_(1.5)/ΣRE 0.600 0.600 0.600 0.600 0.600 0.600 ΣRE/NWF 0.546 0.546 0.558 0.572 0.558 0.552 2LiO_(0.5) + ZnO + (F/2) 28.65 28.40 29.31 29.64 29.61 30.29 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.333 0.335 0.333 0.334 0.327 0.322 ZnO + 3ZrO₂ + 5TaO_(2.5) 44.37 43.88 45.38 45.89 44.69 35.95 Volatilization peak Tv (° C.) (Note 4) 1121 1043 1157 1140 1036 1176 Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 26.33 26.03 26.92 25.31 26.91 26.28 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.41 0.41 0.41 0.41 0.38 0.46 ZrO₂ + TaO_(2.5) 7.44 7.36 7.61 7.70 7.27 5.30 Example Example 20 Example 21 Example 22 Example 23 Example 24 Example 25 BO_(1.5) 36.57 35.58 36.82 36.81 36.82 36.82 SiO₂ 7.41 8.60 8.47 8.47 8.47 8.47 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 2.78 3.23 3.17 1.06 3.17 3.17 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 11.67 13.55 13.33 15.45 13.33 13.33 LaO_(1.5) 22.08 19.84 19.52 19.52 25.87 22.70 GdO_(1.5) 14.72 13.23 13.02 13.02 6.67 9.84 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 1.99 2.31 2.28 2.28 2.28 2.28 TaO_(2.5) 2.78 3.66 3.39 3.39 3.39 3.39 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 34.26 26.88 23.28 23.28 23.28 23.28 O (Note 3) 131.74 135.67 137.29 138.34 137.29 137.29 F/O(Molar ratio) 0.260 0.198 0.170 0.168 0.170 0.170 Glass transition temperature Tg (° C.) 553 561 558 578 566 568 Liquidus temperature LT (° C.) 1080 1070 1070 1070 1120 1120 Specific gravity 5.39 5.34 5.30 5.34 5.18 5.24 Viscosity at liquidus temperature 1.7 3.4 4.0 4.4 2.3 2.3 (dPa · s) Refractive index nd 1.79496 1.80384 1.80542 1.80975 1.80677 1.80607 Abbé number νd 47.86 46.36 46.40 46.03 46.26 46.35 nd − (2.25 − 0.01 × νd) 0.024 0.017 0.019 0.020 0.019 0.020 λ80 (nm) 372 375 374 376 377 374 λ70 (nm) 346 350 348 350 351 350 λ5 (nm) 278 279 278 278 278 279 BO_(1.5) × F/O 9.51 7.04 6.26 6.18 6.26 6.26 Excess B—O—B Bonding:D −22.0 −28.9 −26.8 −27.9 −26.8 −26.8 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.203 0.242 0.230 0.230 0.230 0.230 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 43.98 44.18 45.29 45.28 45.29 45.29 ΣRE 36.80 33.07 32.54 32.54 32.54 32.54 LaO_(1.5)/ΣRE 0.600 0.600 0.600 0.600 0.795 0.698 ΣRE/NWF 0.837 0.748 0.719 0.719 0.719 0.718 2LiO_(0.5) + ZnO + (F/2) 34.36 33.45 31.31 29.21 31.31 31.31 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.323 0.323 0.322 0.121 0.322 0.322 ZnO + 3ZrO₂ + 5TaO_(2.5) 31.54 38.78 37.12 39.24 37.12 37.12 Volatilization peak Tv (° C.) (Note 4) 1137 1194 1121 1114 1054 1128 Weight loss (wt %) (Note 5) 0.74 0.61 BO_(1.5) × F/LaO_(1.5) 28.37 24.10 21.96 21.95 16.57 18.88 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.42 0.39 0.40 0.40 0.40 0.40 ZrO₂ + TaO_(2.5) 4.77 5.97 5.67 5.67 5.67 5.67 Example Example 26 Example 27 Example 28 Example 29 Example 30 Example 31 BO_(1.5) 36.82 36.82 36.82 36.82 38.32 38.12 SiO₂ 8.47 8.47 8.47 8.47 8.11 8.07 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 3.17 3.17 3.17 3.17 1.01 1.01 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 13.33 13.33 13.33 13.33 16.83 16.74 LaO_(1.5) 16.35 13.17 17.62 15.71 18.70 18.61 GdO_(1.5) 16.19 19.37 11.75 10.48 12.47 12.41 YO_(1.5) 0.00 0.00 3.17 6.35 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.28 2.28 2.28 2.28 3.04 2.52 TaO_(2.5) 3.39 3.39 3.39 3.39 1.52 2.52 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 23.28 23.28 23.28 23.28 22.30 22.19 O (Note 3) 137.29 137.29 137.29 137.29 136.52 137.34 F/O(Molar ratio) 0.170 0.170 0.170 0.170 0.163 0.162 Glass transition temperature Tg (° C.) 572 569 569 569 580 581 Liquidus temperature LT (° C.) 1120 1120 1120 1120 1130 1120 Specific gravity 5.36 5.42 5.22 5.13 5.20 5.25 Viscosity at liquidus temperature 2.3 2.3 2.3 2.3 2.3 2.5 (dPa · s) Refractive index nd 1.80466 1.80487 1.80155 1.79808 1.79900 1.80284 Abbé number νd 46.49 46.50 46.57 46.75 47.11 46.54 nd − (2.25 − 0.01 × νd) 0.020 0.020 0.017 0.016 0.020 0.018 λ80 (nm) 376 379 379 375 374 375 λ70 (nm) 352 355 350 349 350 350 λ5 (nm) 280 283 280 279 279 279 BO_(1.5) × F/O 6.26 6.26 6.26 6.26 6.25 6.18 Excess B—O—B Bonding:D −26.8 −26.8 −26.8 −26.8 −21.6 −23.0 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.230 0.230 0.230 0.230 0.212 0.212 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 45.29 45.29 45.29 45.29 46.43 46.19 ΣRE 32.54 32.54 32.54 32.54 31.17 31.02 LaO_(1.5)/ΣRE 0.502 0.405 0.542 0.483 0.600 0.600 ΣRE/NWF 0.718 0.719 0.718 0.719 0.671 0.672 2LiO_(0.5) + ZnO + (F/2) 31.31 31.31 31.31 31.31 30.00 29.85 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.322 0.322 0.322 0.322 0.107 0.108 ZnO + 3ZrO₂ + 5TaO_(2.5) 37.12 37.12 37.12 37.12 33.55 36.90 Volatilization peak Tv (° C.) (Note 4) 1061 1148 1084 1106 1024 1020 Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 26.21 32.54 24.32 27.28 22.85 22.73 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.40 0.40 0.40 0.40 0.67 0.50 ZrO₂ + TaO_(2.5) 5.67 5.67 5.67 5.67 4.56 5.04 Example Example 32 Example 33 Example 34 Example 35 Example 36 Example 37 BO_(1.5) 36.44 42.84 41.57 48.52 48.52 36.66 SiO₂ 8.38 9.79 8.99 1.08 1.08 8.47 GeO₂ 0.00 0.00 0.00 2.16 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 2.16 0.00 LiO_(0.5) 0.00 8.57 3.37 3.23 3.23 2.12 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 1.06 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 17.38 0.00 11.24 12.94 12.94 13.33 LaO_(1.5) 19.32 18.36 17.98 14.88 14.88 19.52 GdO_(1.5) 12.88 12.24 12.36 9.92 9.92 13.02 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.25 3.30 0.00 2.96 2.96 2.34 TaO_(2.5) 3.35 4.90 4.49 4.31 4.31 3.48 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 27.23 20.81 17.98 17.25 17.25 23.28 O (Note 3) 136.36 142.47 141.01 139.09 138.01 137.40 F/O(Molar ratio) 0.200 0.146 0.128 0.124 0.125 0.169 Glass transition temperature Tg (° C.) 588 589 574 568 563 576 Liquidus temperature LT (° C.) 1120 1120 1120 1120 1120 1120 Specific gravity 5.35 5.05 5.18 5.12 5.08 5.28 Viscosity at liquidus temperature 2.2 2.2 2.8 2.2 2.2 2.4 (dPa · s) Refractive index nd 1.80517 1.79086 1.79454 1.80723 1.80365 1.80163 Abbé number νd 46.12 47.05 46.90 45.07 45.56 46.29 nd − (2.25 − 0.01 × νd) 0.016 0.011 0.014 0.008 0.009 0.015 λ80 (nm) 382 371 373 377 382 376 λ70 (nm) 352 348 349 351 358 350 λ5 (nm) 280 281 280 280 286 280 BO_(1.5) × F/O 7.29 6.25 5.32 6.02 6.07 6.20 Excess B—O—B Bonding:D −27.0 −14.0 −16.3 6.5 7.6 −27.4 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.230 0.229 0.216 0.022 0.022 0.231 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 44.82 52.63 50.56 51.76 51.76 45.13 ΣRE 32.20 30.60 30.34 24.80 24.80 32.54 LaO_(1.5)/ΣRE 0.600 0.600 0.593 0.600 0.600 0.600 ΣRE/NWF 0.718 0.581 0.600 0.479 0.479 0.721 2LiO_(0.5) + ZnO + (F/2) 30.99 27.54 26.97 28.03 28.03 29.21 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.000 1.000 0.375 0.333 0.333 0.241 ZnO + 3ZrO₂ + 5TaO_(2.5) 40.88 34.40 33.69 43.37 43.37 37.75 Volatilization peak Tv (° C.) (Note 4) 1081 1050 1037 1079 1092 1106 Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 25.68 24.28 20.79 28.12 28.12 21.86 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.40 0.40 0.00 0.41 0.41 0.40 ZrO₂ + TaO_(2.5) 5.60 8.20 4.49 7.27 7.27 5.82 Example Example 38 Example 39 Example 40 Example 41 Example 42 Example 43 Example 44 BO_(1.5) 36.66 36.28 36.28 42.16 47.62 50.51 47.87 SiO₂ 8.47 8.38 8.38 4.56 4.76 5.05 5.32 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 2.12 3.14 3.14 3.42 4.76 5.05 5.32 NaO_(0.5) 1.06 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 1.05 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 1.05 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 2.28 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 13.33 13.19 13.19 14.36 11.43 6.06 6.38 LaO_(1.5) 19.52 19.32 19.32 16.41 15.24 16.16 17.02 GdO_(1.5) 13.02 12.88 12.88 10.94 9.52 10.10 10.64 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.34 2.31 2.31 2.45 2.86 3.03 3.19 TaO_(2.5) 3.48 3.45 3.45 3.42 3.81 4.04 4.26 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 23.28 23.04 23.04 18.23 7.62 8.08 8.51 O (Note 3) 137.40 137.02 137.02 136.07 143.34 145.96 145.75 F/O(Molar ratio) 0.169 0.168 0.168 0.134 0.053 0.055 0.058 Glass transition temperature 574 570 570 556 577 590 587 Tg (° C.) Liquidus temperature LT (° C.) 1120 1120 1120 1120 1060 1060 1070 Specific gravity 5.29 5.28 5.27 5.16 4.97 4.94 5.04 Viscosity at liquidus temperature 2.4 2.3 2.3 2.4 3.2 3.2 2.9 (dPa · s) Refractive index nd 1.80256 1.80235 1.80368 1.80108 1.80219 1.79967 1.80904 Abbé number νd 46.32 46.48 46.40 46.07 45.83 46.42 45.89 nd − (2.25 − 0.01 × νd) 0.016 0.017 0.018 0.012 0.010 0.014 0.018 λ80 (nm) 373 387 376 378 373 372 375 λ70 (nm) 349 362 350 351 348 348 348 λ5 (nm) 279 299 279 279 281 282 281 BO_(1.5) × F/O 6.20 6.10 6.10 5.65 2.53 2.80 2.80 Excess B—O—B Bonding:D −27.4 −28.2 −28.2 −9.6 −0.5 5.6 −2.1 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.231 0.231 0.231 0.108 0.100 0.100 0.111 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + 45.13 44.66 44.66 46.72 52.38 55.56 53.19 GeO₂) ΣRE 32.54 32.20 32.20 27.35 24.76 26.26 27.66 LaO_(1.5)/ΣRE 0.600 0.600 0.600 0.600 0.616 0.615 0.615 ΣRE/NWF 0.721 0.721 0.721 0.585 0.473 0.473 0.520 2LiO_(0.5) + ZnO + (F/2) 29.21 30.99 30.99 30.32 24.76 20.20 21.28 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.241 0.323 0.323 0.323 0.454 0.625 0.625 ZnO + 3ZrO₂ + 5TaO_(2.5) 37.75 37.37 37.37 38.81 39.06 35.35 37.25 Volatilization peak Tv (° C.) 1059 1027 1104 1046 (Note 4) Weight loss (wt %) (Note 5) 0.20 0.11 0.07 BO_(1.5) × F/LaO_(1.5) 21.86 21.63 21.63 23.42 11.91 12.63 11.97 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.40 0.40 0.40 0.42 0.43 0.43 0.43 ZrO₂ + TaO_(2.5) 5.82 5.76 5.76 5.87 6.67 7.07 7.45 (Note 1) The contents of BO_(1.5) to SbO_(1.5) are given as cationic percentages. (Note 2) F is the relative quantity (in moles) of fluorine ions for the total quantity of cations, denoted as 100. (Note 3) O is the relative quantity (in moles) of oxygen ions for the total quantity of cations, denoted as 100. (Note 4) Volatilization peak Tv is the temperature (° C.) of the endothermic peak accompanying change in the discontinuous weight reduction rate of the glass when heated to 1,200° C. at 10° C./min. (Note 5) The weight reduction was the amount of weight reduction (wt %) when 100 mg of glass was heated from 400° C. to 1,200° C. at 10° C./min.

TABLE 5 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 BO_(1.5) 47.36 47.36 47.36 47.35 47.37 47.36 47.36 48.67 SiO₂ 10.53 10.53 10.53 10.53 8.77 7.02 9.65 9.29 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 14.04 14.04 14.04 14.04 14.04 14.04 14.04 13.72 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.88 LaO_(1.5) 10.53 17.54 19.02 13.60 14.04 14.04 14.04 13.72 GdO_(1.5) 14.03 7.02 4.07 13.60 14.03 14.03 14.03 13.72 YO_(1.5) 3.51 3.51 4.98 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 0.00 0.88 1.75 3.51 0.00 0.00 TaO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.88 0.00 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 14.04 14.04 14.04 14.04 14.04 14.04 14.04 14.16 O (Note 3) 134.210 134.210 134.210 134.650 134.20 134.21 134.65 133.41 F/O(Molar ratio) 0.105 0.105 0.105 0.104 0.105 0.105 0.104 0.106 Glass transition temperature 567 558 556 560 558 557 563 555 Tg (° C.) Liquidus temperature LT (° C.) 1080 1070 1120 1090 1090 1100 1070 1040 Specific gravity(g/cm3) 4.44 4.32 4.12 4.49 4.60 4.66 4.60 4.52 Viscosity at liquidus temperature 1.6 1.8 1.0 1.4 1.4 1.2 1.8 2.4 (dPa · s) Refractive index nd 1.72358 1.72598 1.72510 1.73004 1.74346 1.75760 1.73636 1.72686 Abbé number νd 54.45 54.18 54.25 53.72 52.69 51.19 52.94 54.41 nd − (2.25 − 0.01 × νd) 0.018 0.018 0.018 0.017 0.020 0.020 0.016 0.021 λ80 (nm) 359 357 361 360 370 373 368 356 λ70 (nm) 340 338 342 341 344 346 345 336 λ5 (nm) 278 261 273 279 275 277 278 276 BO_(1.5) × F/O 4.97 4.97 4.97 4.92 4.97 4.97 4.93 5.16 Excess B—O—B Bonding:D 7.9 7.9 7.9 74 7.9 7.9 7.4 12.6 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.222 0.222 0.222 0.222 0.185 0.148 0.204 0.191 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + 57.89 57.89 57.89 57.88 56.14 54.38 57.01 57.96 GeO₂) ΣRE 28.07 28.07 28.07 27.20 28.07 28.07 28.07 27.44 LaO_(1.5)/ΣRE 0.375 0.625 0.678 0.500 0.500 0.500 0.500 0.500 ΣRE/NWF 0.485 0.485 0.485 0.470 0.500 0.516 0.492 0.473 2LiO_(0.5) + ZnO + (F/2) 35.10 35.10 35.10 35.10 35.10 35.10 35.10 35.40 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 1.000 1.000 1.000 1.000 1.000 1.000 1.000 0.969 ZnO + 3ZrO₂ + 5TaO_(2.5) 0.00 0.00 0.00 2.64 5.25 10.53 4.40 0.88 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 31.57 18.95 17.48 24.44 23.69 23.68 23.68 25.12 ZrO₂/(ZrO₂ + TaO_(2.5)) N/A N/A N/A 1.00 1.00 1.00 0.00 N/A ZrO₂ + TaO_(2.5) 0.00 0.00 0.00 0.88 1.75 3.51 0.88 0.00 Example 9 Example 10 Example 11 Example 12 Example 13 Example 14 Example 15 BO_(1.5) 60.11 49.56 48.24 48.89 47.41 48.24 46.43 SiO₂ 0.00 9.46 9.65 9.78 11.21 10.53 10.71 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 8.20 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 14.21 12.61 12.28 12.44 11.21 11.40 14.29 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 1.79 SrO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 0.00 0.00 2.63 0.89 3.45 2.63 0.00 LaO_(1.5) 8.74 13.96 13.60 13.78 13.36 13.60 13.39 GdO_(1.5) 8.74 13.96 13.60 13.78 13.36 13.60 13.39 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.45 0.00 0.44 0.00 0.00 0.00 TaO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 100.00 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 8.74 14.41 17.54 14.22 13.79 14.04 10.71 O (Note 3) 131.42 135.14 132.46 135.12 135.78 135.53 134.82 F/O(Molar ratio) 0.067 0.107 0.132 0.105 0.102 0.104 0.079 Glass transition temperature Tg (° C.) 547 558 553 566 562 565 556 Liquidus temperature LT (° C.) 1040 1050 1040 1040 1050 1040 1100 Specific gravity(g/cm3) 3.86 4.54 4.55 4.54 4.53 4.54 4.46 Viscosity at liquidus temperature 2.0 2.4 2.8 2.8 3.0 3.0 1.4 (dPa · s) Refractive index nd 1.68084 1.73062 1.72482 1.73085 1.72791 1.72955 1.72912 Abbé number νd 56.60 54.16 54.42 54.06 54.00 54.04 53.82 nd − (2.25 − 0.01 × νd) −0.003 0.022 0.019 0.021 0.018 0.020 0.017 λ80 (nm) 371 356 354 356 353 354 360 λ70 (nm) 355 336 334 338 336 336 339 λ5 (nm) 298 277 276 277 277 277 274 BO_(1.5) × F/O 4.03 5.30 6.37 5.13 4.84 5.02 3.67 Excess B—O—B Bonding:D 48.9 13.5 12.3 11.6 6.4 9.2 4.5 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.000 0.191 0.200 0.200 0.236 0.218 0.231 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 68.31 59.02 57.89 58.67 58.62 58.77 57.14 ΣRE 17.48 27.92 27.20 27.56 26.72 27.20 26.78 LaO_(1.5)/ΣRE 0.500 0.500 0.500 0.500 0.500 0.500 0.500 ΣRE/NWF 0.256 0.473 0.470 0.470 0.456 0.463 0.469 2LiO_(0.5) + ZnO + (F/2) 32.79 32.43 35.96 32.88 32.77 32.45 33.94 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 1.000 1.000 0.903 0.965 0.867 0.897 1.000 ZnO + 3ZrO₂ + 5TaO_(2.5) 0.00 1.35 2.63 2.21 3.45 2.63 0.00 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 30.06 25.58 31.11 25.23 24.47 24.90 18.57 ZrO₂/(ZrO₂ + TaO_(2.5)) N/A 1.00 N/A 1.00 N/A N/A N/A ZrO₂ + TaO_(2.5) 0.00 0.45 0.00 0.44 0.00 0.00 0.00 Example 16 Example 17 Example 18 Example 19 Example 20 Example 21 Example 22 BO_(1.5) 46.43 46.43 46.52 41.80 61.82 56.03 42.48 SiO₂ 10.71 10.71 10.08 14.55 4.55 7.25 14.16 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 14.29 14.29 12.40 14.55 7.27 8.70 14.16 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 3.64 5.80 0.00 SrO 1.79 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 1.79 0.00 0.00 0.00 0.00 0.00 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 LaO_(1.5) 13.39 13.39 15.50 14.55 12.27 16.91 14.60 GdO_(1.5) 13.39 13.39 15.50 14.55 10.45 4.83 14.60 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.48 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 TaO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 10.71 10.71 24.79 14.55 10.91 11.59 10.62 O (Note 3) 134.82 134.82 130.25 135.45 137.73 136.23 137.61 F/O(Molar ratio) 0.079 0.079 0.190 0.107 0.079 0.085 0.077 Glass transition temperature Tg (° C.) 561 557 554 561 599 578 574 Liquidus temperature LT (° C.) 1110 1110 1060 1080 1070 1070 1120 Specific gravity(g/cm3) 4.48 4.50 4.70 4.55 4.17 3.98 4.57 Viscosity at liquidus temperature 1.2 1.2 2.0 1.6 1.6 1.8 1.2 (dPa · s) Refractive index nd 1.72923 1.72886 1.72964 1.72849 1.70816 1.70373 1.73803 Abbé number νd 53.90 53.75 54.65 53.88 56.25 56.07 53.06 nd − (2.25 − 0.01 × νd) 0.018 0.016 0.026 0.017 0.021 0.014 0.019 λ80 (nm) 361 363 358 357 358 358 364 λ70 (nm) 342 343 339 337 339 341 345 λ5 (nm) 275 277 274 278 278 278 279 BO_(1.5) × F/O 3.67 3.67 8.84 4.47 4.88 4.76 3.27 Excess B—O—B Bonding:D 4.5 4.5 9.3 −10.1 47.7 31.9 −10.2 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.231 0.231 0.217 0.348 0.074 0.129 0.333 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 57.14 57.14 56.60 56.35 66.37 63.28 56.64 ΣRE 26.78 26.78 31.00 29.10 22.72 22.22 29.20 LaO_(1.5)/ΣRE 0.500 0.500 0.500 0.500 0.540 0.761 0.500 ΣRE/NWF 0.469 0.469 0.548 0.516 0.342 0.351 0.516 2LiO_(0.5) + ZnO + (F/2) 33.94 33.94 37.20 36.37 19.99 23.20 33.63 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 1.000 1.000 1.000 1.000 1.000 1.000 1.000 ZnO + 3ZrO₂ + 5TaO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) BO_(1.5) × F/LaO_(1.5) 18.57 18.57 37.20 20.90 27.48 19.20 15.45 ZrO₂/(ZrO₂ + TaO_(2.5)) N/A N/A N/A N/A N/A N/A N/A ZrO₂ + TaO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Example 23 Example 24 Example 25 Example 26 Example 27 Example 28 BO_(1.5) 44.84 63.58 51.03 62.06 60.71 63.75 SiO₂ 10.34 2.89 3.06 2.66 2.60 2.73 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 13.79 15.03 12.24 7.09 6.94 7.29 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 5.17 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 0.00 0.00 5.61 2.66 2.60 0.00 LaO_(1.5) 12.93 9.25 13.27 13.56 13.27 13.93 GdO_(1.5) 12.93 6.95 11.22 11.97 11.71 12.30 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 0.00 0.00 3.06 0.00 2.17 0.00 TaO_(2.5) 0.00 0.00 0.51 0.00 0.00 0.00 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 0.00 0.00 0.00 0.00 0.00 0.00 YbO_(1.5) 0.00 2.30 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 10.34 9.25 20.41 10.64 10.41 10.93 O (Note 3) 133.63 131.79 128.32 137.59 138.94 138.61 F/O(Molar ratio) 0.077 0.070 0.159 0.077 0.075 0.079 Glass transition temperature Tg (° C.) 550 566 519 590 591 604 Liquidus temperature LT (° C.) 1090 1040 1070 1070 1060 1070 Specific gravity(g/cm3) 4.41 4.54 4.61 4.50 4.52 4.46 Viscosity at liquidus temperature 1.4 3.0 3.0 3.0 3.3 3.0 (dPa · s) Refractive index nd 1.72536 1.68597 1.74343 1.73570 1.74675 1.73115 Abbé number νd 53.93 57.31 51.52 54.17 52.64 54.83 nd − (2.25 − 0.01 × νd) 0.015 0.009 0.009 0.027 0.023 0.029 λ80 (nm) 379 354 358 361 359 358 λ70 (nm) 362 336 338 340 338 337 λ5 (nm) 312 277 265 275 275 275 BO_(1.5) × F/O 3.45 4.45 8.12 4.80 4.55 5.03 Excess B—O—B Bonding:D 0.9 59.0 24.8 48.6 43.2 52.6 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.231 0.045 0.060 0.043 0.043 0.043 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 55.18 66.47 54.09 64.72 63.31 66.48 ΣRE 25.86 18.50 24.49 25.53 24.98 26.23 LaO_(1.5)/ΣRE 0.500 0.500 0.542 0.531 0.531 0.531 ΣRE/NWF 0.469 0.278 0.453 0.395 0.395 0.395 2LiO_(0.5) + ZnO + (F/2) 32.75 34.68 40.29 22.16 21.68 20.04 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 1.000 1.000 0.814 0.842 0.842 1.000 ZnO + 3ZrO₂ + 5TaO_(2.5) 0.00 0.00 17.34 2.66 9.11 0.00 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 0.69 0.78 0.71 BO_(1.5) × F/LaO_(1.5) 17.93 31.79 39.24 24.35 23.81 25.01 ZrO₂/(ZrO₂ + TaO_(2.5)) N/A N/A 0.86 N/A 1.00 N/A ZrO₂ + TaO_(2.5) 0.00 0.00 3.57 0.00 2.17 0.00 (Note 1) The contents of BO_(1.5) to SbO_(1.5) are given as cationic percentages. (Note 2) F is the relative quantity (in moles) of fluorine ions for the total quantity of cations, denoted as 100. (Note 3) O is the relative quantity (in moles) of oxygen ions for the total quantity of cations, denoted as 100. (Note 4) Volatilization peak Tv is the temperature (° C.) of the endothermic peak accompanying change in the discontinuous weight reduction rate of the glass when heated to 1,200° C. at 10° C./min. (Note 5) The weight reduction was the amount of weight reduction (wt %) when 100 mg of glass was heated from 400° C. to 1,200° C. at 10° C./min.

TABLE 6 Example Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 BO_(1.5) 35.61 34.96 37.00 36.29 33.11 42.00 SiO₂ 1.87 1.84 1.95 1.91 3.89 0.00 GeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 LiO_(0.5) 3.75 3.68 3.89 3.82 3.89 0.00 NaO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 0.00 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 BaO 0.00 0.00 0.00 0.00 0.00 0.00 ZnO 17.81 17.48 18.50 18.15 18.50 19.00 LaO_(1.5) 20.62 21.16 19.47 20.06 20.45 19.00 GdO_(1.5) 7.97 8.74 6.33 7.16 7.30 6.00 YO_(1.5) 0.00 0.00 0.00 0.00 0.00 1.00 TiO₂ 0.00 0.00 0.00 0.00 0.00 0.00 ZrO₂ 2.53 2.48 2.63 2.58 2.63 2.00 TaO_(2.5) 5.15 5.06 5.36 5.25 5.36 4.00 NbO_(2.5) 0.00 0.00 0.00 0.00 0.00 0.00 WO₃ 4.69 4.60 4.87 4.78 4.87 7.00 YbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 100.00 100.00 100.00 F (Note 2) 11.25 14.72 3.89 7.64 7.79 8.00 O (Note 3) 146.11 144.34 149.87 147.95 148.89 152.00 F/O(Molar ratio) 0.077 0.102 0.026 0.052 0.052 0.053 Glass transition temperature Tg (° C.) 562 559 568 564 567 585 Liquidus temperature LT (° C.) 1120 1120 1120 1120 1120 1120 Specific gravity 5.65 5.69 5.53 5.61 5.61 5.61 Viscosity at liquidus temperature 1.6 1.4 2.0 1.8 2.0 1.8 (dPa · s) Refractive index nd 1.87924 1.87679 1.88262 1.88077 1.88283 1.87398 Abbé number νd 37.96 38.19 37.34 37.70 37.44 36.99 nd − (2.25 − 0.01 × νd) 0.009 0.009 0.006 0.008 0.007 −0.006 λ80 (nm) 443 442 443 448 451 438 λ70 (nm) 386 386 385 385 387 387 λ5 (nm) 345 344 347 347 347 350 BO_(1.5) × F/O 2.74 3.57 0.96 1.89 1.72 2.23 Excess B—O—B Bonding:D −39.3 −39.5 −38.9 −39.1 −49.6 −26.0 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.053 0.053 0.053 0.053 0.117 0.000 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 37.48 36.80 38.95 38.20 37.00 42.00 ΣRE 28.59 29.90 25.80 27.22 27.75 26.00 LaO_(1.5)/ΣRE 0.721 0.708 0.755 0.737 0.737 0.731 ΣRE/NWF 0.763 0.812 0.662 0.712 0.750 0.619 2LiO_(0.5) + ZnO + (F/2) 30.93 32.20 28.23 29.61 30.17 23.00 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.296 0.296 0.296 0.296 0.296 0.000 ZnO + 3ZrO₂ + 5TaO_(2.5) 51.15 50.22 53.19 52.14 53.19 45.00 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 0.51 BO_(1.5) × F/LaO_(1.5) 9.71 12.16 3.7 6.91 6.31 8.84 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.33 0.33 0.33 0.33 0.33 0.33 ZrO₂ + TaO_(2.5) 7.68 7.54 7.99 7.83 7.99 6.00 Example Example 7 Example 8 Example 9 BO_(1.5) 32.00 38.10 36.29 SiO₂ 6.00 3.81 1.91 GeO₂ 0.00 0.00 0.00 AlO_(1.5) 0.00 0.00 0.00 LiO_(0.5) 0.00 3.81 3.82 NaO_(0.5) 0.00 0.00 0.00 KO_(0.5) 0.00 0.00 0.00 MgO 0.00 0.00 0.00 CaO 0.00 0.00 0.00 SrO 0.00 0.00 0.00 BaO 0.00 0.00 0.00 ZnO 21.00 15.24 18.15 LaO_(1.5) 21.50 20.95 20.06 GdO_(1.5) 5.00 7.62 7.16 YO_(1.5) 0.00 0.00 0.00 TiO₂ 0.50 0.00 0.00 ZrO₂ 2.50 2.86 2.58 TaO_(2.5) 5.00 5.71 5.25 NbO_(2.5) 0.50 0.00 0.00 WO₃ 6.00 1.90 4.78 YbO_(1.5) 0.00 0.00 0.00 CeO₂ 0.00 0.00 0.00 SnO₂ 0.00 0.00 0.00 SbO_(1.5) 0.00 0.00 0.00 Total (Note 1) 100.00 100.00 100.00 F (Note 2) 8.00 11.43 7.64 O (Note 3) 154.50 144.75 147.95 F/O(Molar ratio) 0.052 0.079 0.052 Glass transition temperature Tg (° C.) 597 567 560 Liquidus temperature LT (° C.) 1120 1100 1110 Specific gravity 5.61 5.44 5.60 Viscosity at liquidus temperature 2.0 2.2 2.0 (dPa · s) Refractive index nd 1.88583 1.85348 1.87976 Abbé number νd 36.33 40.91 37.67 nd − (2.25 − 0.01 × νd) −0.001 0.013 0.006 λ80 (nm) 441 405 463 λ70 (nm) 388 372 387 λ5 (nm) 351 336 346 BO_(1.5) × F/O 1.66 3.01 1.87 Excess B—O—B Bonding:D −58.5 −30.5 −39.1 (BBO1.5-(Boxygen-BBO1.5)) SiO₂/BO_(1.5) 0.188 0.100 0.053 NWF (BO_(1.5) + SiO₂ + AlO_(1.5) + GeO₂) 38.00 41.91 38.20 ΣRE 26.50 28.57 27.22 LaO_(1.5)/ΣRE 0.811 0.733 0.737 ΣRE/NWF 0.697 0.682 0.712 2LiO_(0.5) + ZnO + (F/2) 25.00 28.57 29.61 2LiO_(0.5)/(2LiO_(0.5) + ZnO) 0.000 0.333 0.296 ZnO + 3ZrO₂ + 5TaO_(2.5) 53.50 52.37 52.14 Volatilization peak Tv (° C.) (Note 4) Weight loss (wt %) (Note 5) 0.23 0.06 BO_(1.5) × F/LaO_(1.5) 5.95 10.39 6.91 ZrO₂/(ZrO₂ + TaO_(2.5)) 0.33 0.33 0.33 ZrO₂ + TaO_(2.5) 7.50 8.57 7.83 (Note 1) The contents of BO_(1.5) to SbO_(1.5) are given as cationic percentages. (Note 2) F is the relative quantity (in moles) of fluorine ions for the total quantity of cations, denoted as 100. (Note 3) O is the relative quantity (in moles) of oxygen ions for the total quantity of cations, denoted as 100. (Note 4) Volatilization peak Tv is the temperature (° C.) of the endothermic peak accompanying change in the discontinuous weight reduction rate of the glass when heated to 1,200° C. at 10° C./min. (Note 5) The weight reduction was the amount of weight reduction (wt %) when 100 mg of glass was heated from 400° C. to 1,200° C. at 10° C./min.

The value of B_(B)—(B_(O)—B_(B)) was denoted as D, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O). In Example 32 of Table 3, D is −0.60. In Example 33, D is 0.60. Glass in which D was 22.0 was prepared as a reference sample (reference composition).

Changes in the refractive index nd and the Abbé number νd when the melting time was varied were examined for Examples 32 and 33 in Table 3 and the reference composition. The results are given in FIG. 1.

The numbers recorded near the various plots in FIG. 1 denote melting times in unit of hours. Examples 32 and 33 of Table 3, in which D was small, exhibited smaller changes in refractive index and Abbé number for change in melting time than the reference composition.

FIG. 2 shows the change in refractive index in Examples 32 and 33 of Table 3, with the melting time denoted by the horizontal axis and the refractive index nd denoted by the vertical axis.

FIG. 3 shows the change in specific gravity in Examples 32 and 33 of Table 3, with the melting time denoted by the horizontal axis and the specific gravity denoted by the vertical axis.

FIG. 4 shows the change in the glass transition temperature in Example 32 of Table 3, with the melting time denoted by the horizontal axis and the glass transition temperature Tg denoted by the vertical axis.

From these results, it will be understood that the change in characteristics accompanying change in the melting time was smaller in Examples 32 and 33 of Table 3, which had a lower D than in the reference composition, which had a D of 22.0.

When D increased above 22.0, and when it exceeded 60, the change in characteristics accompanying the change in melting time became extremely large. This meant that in the course of preparing the glass, the variation in the various characteristics of the glass ended up increasing. The main factor behind the variation in characteristics was thought to be volatilization from the glass. With pronounced volatilization, striae tended to form in the molded glass.

Additionally, since the value of D was low in the optical glasses of the examples of the present invention, including Examples 32 and 33 of Table 3, volatilization was suppressed, striae tended not to form, and there was little variation in the various characteristics during large-quantity production.

Keeping D within the above desirable range made it possible to control volatilization of the glass, preventing a decrease in quality due to variation in characteristics and striae.

The optical glasses thus obtained were suitable as glass materials for use in precision press molding.

The various characteristics of the optical glasses were measured by the following methods.

(1) The Refractive Index nd and the Abbé Number νd

The refractive indexes nd, nF, and nc of the glasses obtained by lowering the temperature at a rate of −30° C./hour were measured by the refractive index measuring methods of the Japan Optical Glass Industry Association Standards. The Abbé number νd was computed from these results.

(2) The Liquidus Temperature LT and the Viscosity at Liquidus Temperature

The glass was placed in a furnace that had been heated to a predetermined temperature, kept there for two hours, and cooled. The interior of the glass was then observed by optical microscopy at 100-fold magnification to determine the liquidus temperature based on the presence or absence of crystals.

The viscosity of the glass at the liquidus temperature was measured by the rotating cylinder method based on “JIS Z 8803-1991 ‘Liquid Viscosity—Measurement Methods,’ 8. Viscosity measurement by single cylinder type rotational viscometer.”

(3) The Glass Transition Temperature Tg

The glass transition temperature was measured at a heating rate of 10° C./min with a differential scanning calorimeter (DSC3300) made by Bruker ASX.

(4) Specific Gravity

The specific gravity was measured by Archimedes' method.

(5) λ(lambda) 80, λ70, λ5 (nm)

Plate like glass samples that were 1.0 mm in thickness, had two mutually parallel surfaces, and had been optically polished were employed. Monochromatic light of intensity I_(in) was made to perpendicularly enter one of the optically polished surfaces, pass through the sample, and exit through the other optically polished surface, at which time the intensity was denoted as I_(out). The external transmittance I_(out)/I_(in) was then calculated. The external transmittance was measured over a wavelength range of 280 to 700 nm. The wavelength at which the external transmittance was 80 percent was denoted as λ80, that at which the external transmittance was 70 percent as λ70, and that at which the external transmittance was 5 percent as λ5.

(6) The Methods of Measuring Change Over Time

-   (i) Refractive index nd -   (ii) Abbé number νd -   (iii) Specific gravity -   (iv) Glass transition temperature Tg

The glass melt was sampled during melting and glass samples were used to measure the various characteristics by the above-described methods. A certain interval of time following the sampling was allowed to elapse, the glass melt was again sampled, and the glass samples were used to measure the various characteristics. The change in various characteristics over time was computed from measured values of the various characteristics obtained by two or more samplings and the sampling time interval.

(7) Weight Reduction

The weight reduction indicates the reduction in weight of fluorine-containing glass as glass components volatize during heating. To quantify the level of weight reduction, equipment measuring the change in weight of a sample accompanying change in temperature, such as TG-DTA measurement and TG-MS measurement, can be employed.

In TG-DTA, for example, a glass sample positioned on a precision scale is heated and the weight of the sample at various temperatures is measured to quantify the change in weight of the glass as the temperature rises.

Specifically, a prescribed weight W (about 100 mg) of glass sample in powder form was packed in a Pt cell 5 mm in diameter and 5 mm in height and heated at a rate of 10° C./min to 1,200° C. with a Rigaku Denki TG-DTA (Thermo Plus TG8120). The amount of weight reduction in the glass, Δ(DELTA)W (mg) was then measured, and the weight reduction was calculated as Weight reduction=ΔW/W×100(%).

Since weight reduction that is unrelated to the volatilization of glass components is sometimes observed due to the elimination of moisture and organic material adhering to the glass over the temperature range of from room temperature to 400° C., the weight reduction of the glass was defined as the weight reduction from 400 to 1,200° C.

(8) Volatilization Peak

The volatilization peak is the temperature of the sharp endothermic peak that appears in the high temperature range as the result of measurement by differential scanning calorimetry, DSC. That is, in the TG-DTA measurement of fluorine-containing glass, at temperatures exceeding 900° C., the rate of weight reduction of glass components with temperature change increases discontinuously, and displacement of heat accompanying the absorption of heat by the glass is sometimes observed. With such change in the weight reduction behavior of the glass, the temperature at which the peak in endothermic behavior is exhibited is defined in the present invention as the volatilization peak temperature Tv of the glass. The endothermic peak at temperature Tv can be measured by differential scanning calorimetry DSC if the range of change in the weight reduction rate is specified in advance by TG-DTA measurement.

Pronounced volatilization occurs at and above Tv. The lower temperature limit of the temperature range at which crystals that have precipitated melt is the liquidus temperature LT. So long as the glass melt is maintained at or above LT, crystallization (devitrification) will not occur. If Tv is higher than LT, the glass melt can be molded at a temperature between LT and Tv. Thus, volatilization can be prevented while preventing devitrification. The greater the temperature differential between LT and Tv, the wider the temperature range over which devitrification and volatilization can be prevented, and the easier it is to stably provide high-quality glass.

As the specific magnitude of the rate of weight reduction with temperature change, for a glass weight reduction of 0.5 percent, for example, the rate of weight reduction with temperature change of the glass is about 1×10⁻³ wt %/° C. below or equal to temperature Tv, becoming about 3×10⁻³ wt %/° C. above or equal to temperature Tv. For a glass weight reduction of 2 percent, the rate of weight reduction with temperature change is about 1×10⁻³ wt %/° C. below or equal to temperature Tv, becoming about 15×10⁻³ wt %/° C. above or equal to temperature Tv.

When LT≦Tv does not hold true, it does not mean that it is impossible to simultaneously prevent both devitrification and volatilization. Since it takes time for crystals to precipitate when maintaining the glass melt at a temperature somewhat lower than LT, volatilization does not occur. (The lower the temperature that is being maintained, the more rapidly crystal precipitation occurs.) Thus, the desirable range of Tv in Form B (Table 4) contains a temperature range that is lower than LT.

Comparative Examples

The glass of Example 5 described in Patent Document 1 (Japanese Unexamined Patent Publication (KOKAI) No. 2005-170782) was prepared by the method described in Patent Document 1. After maintaining the glass at 1,140° C. for two hours, an optical microscope was used to observe whether or not crystals had precipitated. FIG. 7 shows a microscopic photograph of the glass after it had been maintained at this temperature. As will be clear from FIG. 7, crystals were found on the surface and in the interior of the glass. These results clearly indicate that the liquidus temperature of Example 5 described in Patent Document 1 (Japanese Unexamined Patent Publication (KOKAI) No. 2005-170782) was higher than 1,140° C.

Wet Backwordly Comparison Test

The glass (Form B) of Example 22 in Table 4 and the glasses of Examples 2 and 3 in Patent Document 1 were subjected to a wet backwordly comparison test conducted by the following method. The results (photographs) are given in FIG. 8. As a result, it will be understood that while the glass of the present invention exhibited no wet backwordly, the glasses of Examples 2 and 3 of Patent Document 1 exhibited pronounced wet backwordly as well as high volatility.

Transmittance Curve

FIG. 9 shows transmittance curves that are the results of measurement of the spectral transmittance of samples 10 mm in thickness of the following glasses that were reproduced:

-   (1) Example 31 (YbO_(1.5)=0 percent) described in Patent Document 3     (WO2004/054937); -   (2) Example 5 (YbO_(1.5)=0 percent) of Patent Document 1 (Japanese     Unexamined Patent Publication (KOKAI) No. 2005-170782); -   (3) Example 34 (YbO_(1.5)=6.11 percent); -   (4) Example 3 (YbO_(1.5)=1 percent); -   (5) Example 2 (YbO_(1.5)=0.98 percent).

The glasses of (1) and (2) above that did not contain YbO_(1.5), described in Patent Documents 1 and 3, had high transmittance over a wavelength range of 900 to 1,000 nm. However, the glasses of (4) and (5), which contained slight amounts of YbO_(1.5), exhibited transmittance of less than 20 percent over the same wavelength range because of strong absorbance by Yb. Almost no light of the above wavelength passed through the glass of (3). Thus, glasses containing more than 0.5 percent of YbO_(1.5) are not suitable as materials for optical elements constituting image pickup optical systems in which high transmittance in the infrared range is desirable.

The various above glasses were then ground and polished to prepare precision press-molding preforms. It was possible for the shape of the preforms to be close to the shape of the optical element to be fabricated, spherical, or some other shape.

The entire surface of the preforms thus obtained was coated with a carbon film as necessary. The preforms were heated and precision press molded in a pressing mold to fabricate aspherical lenses such as convex meniscus lenses, concave meniscus lenses, biconvex lenses, biconcave lenses, plano-convex lenses, and plano-concave lenses. Following precision press molding, the molded products were centered as needed to finish the lenses. An antireflective film was also coated as needed on the surfaces of the lenses obtained.

Instead of a SiC mold, it was also possible to conduct precision press molding with a WC mold or a cermet mold. A mold release film in the form of a carbon film could also be coated on the molding surfaces of a SiC mold. A noble metal film, such as platinum, or a noble metal alloy film could also be coated on the molding surfaces of a WC mold.

Due to the low glass transition temperature of the optical glass, it was possible to lower the precision press molding temperature. Thus, good precision press molding could be conducted even without using a mold with extremely good heat resistance such as a SiC mold.

Next, the interchangeable lenses of a single-lens reflex camera and the image pickup optical system of a compact camera that incorporated the various above-described aspherical lenses were fabricated.

Further, the CCDs or CMOSs of the image pickup optical system of a compact camera were assembled to fabricate an image pickup optical system unit, which was built into a compact camera.

INDUSTRIAL APPLICABILITY

The present invention is useful in fields relating to optical glass. 

1. An optical glass comprising, denoted as cationic percentages: BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 20 percent; ZrO₂ 0 to 4.5 percent; TaO_(2.5) 0 to 7 percent; YbO_(1.5) 0 to 0.5 percent; GeO₂ 0 to 5 percent; YO_(1.5) 0.5 to 10 percent;

as well as O and F, with the F/O molar ratio of the F content to the O content being 0.01 to 0.30; the combined sum of the ZnO content, three times the ZrO₂ content, and five times the TaO_(2.5) content (ZnO+3ZrO₂+5TaO_(2.5)) being 40 percent or less; the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater; the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to +20, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O); the refractive index nd being 1.75 to 1.81; the Abbé number νd being 48 to 52; and the glass transition temperature Tg being 590° C. or lower.
 2. The optical glass according to claim 1, wherein the refractive index nd and the Abbé number νd of the optical glass satisfy equation (1) below nd−(2.25−0.01×νd)≧−0.01  (1).
 3. An optical glass comprising, denoted as cationic percentages: BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 25 percent; ZrO₂ 8 percent or less; TaO_(2.5) 7 percent or less; YbO_(1.5) 0 to 3 percent; GeO₂ 0 to 5 percent;

as well as O and F, with the F/O molar ratio of the F content to the O content being 0.01 to 0.30; the ratio of the ZrO₂ content to the combined content of ZrO₂ and TaO_(2.5) (ZrO₂/(ZrO₂+TaO_(2.5))) being 0 to 0.7; the combined sum of ZrO₂ and TaO_(2.5) (ZrO₂+TaO_(2.5)) being 3 to 10; the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater; the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to +20, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O); the refractive index nd being 1.802 to 1.835; and the Abbé number νd being 42 to
 48. 4. The optical glass according to claim 3, wherein the refractive index nd and the Abbé number νd of the optical glass satisfy equation (2) below nd−(2.25−0.01×νd)≧−0.01  (2).
 5. An optical glass comprising, denoted as cationic percentages: BO_(1.5) 20 to 70 percent; LaO_(1.5) 6 to 30 percent; GdO_(1.5) 4 to 25 percent; SiO₂ 0 to 20 percent; LiO_(0.5) 0 to 20 percent; ZnO 0 to 15 percent; ZrO₂ 5 percent or less; TaO_(2.5) 3 percent or less; YbO_(1.5) 0 to 3 percent; GeO₂ 0 to 5 percent;

as well as O and F, with the F/O molar ratio of the F content to the O content being 0.01 to 0.30; the combined sum of the ZnO content, three times the ZrO₂ content, and five times the TaO_(2.5) content (ZnO+3ZrO₂+5TaO_(2.5)) being 15 percent or less; the combined sum of twice the LiO_(0.5) content, the ZnO content, and ½ the F content (2LiO_(0.5)+ZnO+(F/2)) when the total quantity of cations is denoted as 100 being 20 percent or greater; the value of B_(B)—(B_(O)—B_(B)) falling within a range of −60 to +60, with the value B_(B) being 1.5 times the content of BO_(1.5) denoted as a cationic percentage, the total quantity of all cations in the glass being denoted as 100, and the molar ratio of the total oxygen content relative to this total quantity being denoted as B_(O); the refractive index nd being 1.675 to 1.745; and the Abbé number νd being 51 to
 58. 6. The optical glass according to claim 5, wherein the refractive index nd and the Abbé number νd of the optical glass satisfy equation (3) below nd−(2.25−0.01×νd)≧−0.01  (3).
 7. A preform for precision press molding that is comprised of an optical glass according to claim
 1. 8. An optical element that is comprised of an optical glass according to claim
 1. 9. A method of manufacturing an optical element comprises heating and precision press molding a preform according to claim 7 with a pressing mold.
 10. A lens unit being equipped with an optical element according to claim
 8. 11. An image pickup device being equipped with a lens unit of according to claim
 10. 